Glossary
A
- ACID ACID is four rules that keep a database write trustworthy: atomicity, consistency, isolation, and durability. In a ledger they mean a post either lands on both sides or on neither, the book stays balanced, two posts cannot corrupt the same balance, and a committed write survives a crash. A payment API that returns 200 is not the same thing as an ACID apply.
- AML AML is anti-money laundering: the program that watches how money moves after you onboard someone. KYC asks who they are. AML asks whether the activity looks like crime. A ledger does not decide that. It gives you the history an AML system reads: amounts, counterparties, velocity, and who owns the wallet.
- Account normality Account normality is which side increases a balance. Debit-normal accounts rise on debit and fall on credit. Credit-normal accounts do the reverse. Assets and expenses are usually debit-normal. Liabilities, equity, and revenue are usually credit-normal. Source and destination name the path of the money. Normality is how each side of that path is signed.
- Agentic payments Agentic payments are transfers an AI agent originates. The agent decides to pay, then calls your API or MCP. The ledger does not grow a new object for that. The agent is a client. It still needs a mandate, a unique reference, and the same queue and locks as a human click. What changes is how sloppy the caller can be.
B
- Balance A balance is a store of value in the ledger: a wallet, a bank account, a card, points, escrow, or an internal account such as revenue. Every transaction moves between two balances. The number you show is the sum of those movements.
- Balance caching Balance caching means you store the current total on the balance instead of summing the whole log on every read. The cache is the running number. The log is still the source of history. A cache you keep in Redis and never write back to the book is a second ledger, and it will lie.
C
- Chart of accounts A chart of accounts is the named list of balances your product will post to. Each name is a bucket: wallets, clearing, fees, revenue, payouts. You draw it before you write posting code. If a movement has no account on the chart, you do not have a place to put it.
- Clearing account A clearing account is a temporary balance that holds money between two real homes. You debit the wallet and credit clearing when a payout starts. You debit clearing when the rail pays. If clearing is not zero at the end of the day, something is still open or something broke.
- Client money account A client money account holds funds that belong to customers, not to you. Safeguarding and client-money rules say you must keep that pot separate from operating cash and be able to show who owns it. The account is at a bank. The split is in your ledger.
D
- Distributed lock A distributed lock is a lock held outside the database so two workers cannot apply conflicting posts at the same time. The ledger locks the balances in a request, in a fixed order, then reads and writes. If it cannot get every lock, it releases what it has and retries. Without the order, two transfers deadlock.
- Double-entry Double-entry means every transaction has two sides: a debit and a credit of the same amount. In product language those sides are source and destination. The book stays balanced because you never post one side alone. A fee, a deposit, and a refund all use that pair.
E
F
- FBO account An FBO account is a bank account titled “for the benefit of” your customers. The bank holds the funds. Your ledger says who owns which part. The title is a custody structure, not a product feature. Without a per-customer book, the FBO is only a pooled pot with a legal name on it.
- Float Float is money that has left one balance and has not yet landed on another. It exists because rails take time. The sender is down. The receiver is not yet up. Someone holds the difference, often a bank or a processor, until settlement.
G
H
- Historical balance A historical balance is the amount a ledger balance held at a time in the past. You ask for a timestamp and get the balance as of that instant. It is a read, not a new account. Teams use it for statements, disputes, and reconstruction.
- Hot balance A hot balance is a ledger balance that takes so many concurrent posts that it becomes a bottleneck. The row is locked, queued work piles up, and latency rises. Engineers often call this a hot row. The balance is the thing that overheats.
I
- Idempotency Idempotency means sending the same request twice produces one movement, not two. In a ledger that is a unique reference on the transaction. Timeouts, retries, and double clicks all resend work. The reference is what stops the second post from becoming a second credit.
- Identity An identity is the customer or organization record you attach to balances. It is not the wallet. It is who the wallet belongs to. One identity can have many balances. A balance without an identity still works. You just cannot ask “who.”
- Immutability Immutability means a posted transaction is not edited. A new status, a refund, or a void is a new record that points at the old one. The history stays in the log. You can reconstruct what the book showed at any time without trusting an overwrite.
- Inflight An inflight transaction is a money movement that is held until you commit or void it. The amount is reserved on inflight balances, and the settled balances do not change. Use it when authorization, verification, or an external rail has to finish before the transfer should apply.
J
K
- KYB KYB is Know Your Business: the checks you run on a company before you hold a balance for it. You identify the legal entity and the people who own or control it. KYC is the person. KYB is the entity. A marketplace that pays out to businesses needs both.
- KYC KYC is Know Your Customer: the checks you run to identify a person and decide if you will hold a balance for them. It is not the wallet. It is the gate in front of the wallet. A ledger can store the identity and the decision. It does not replace a KYC vendor or a CIP policy.
L
- Ledger A ledger is the system of record for money in a product. It stores balances and the transactions that change them. Banks, wallets, and marketplaces each have one, even when they also use a bank. The bank statement is not the product’s book.
- Ledger database A ledger database is the store that holds balances and the transactions that change them. It is built for append-only posts, unique references, and balance invariants. Your application database holds customers and sessions. If you post money in the app database, you have two books, and they will drift.
- Ledger sharding Ledger sharding splits one logical balance into many physical balances so concurrent posts stop locking the same row. Your app picks a shard, posts to it, and sums the shards when it reports the total. The customer still sees one pot. The book has several.
M
- MCP MCP is the Model Context Protocol. It gives an AI agent a typed list of tools instead of a pile of undocumented HTTP. The agent calls those tools the way your app calls an SDK. In a ledger, the useful tools are “get this balance,” “find this reference,” and “record this transfer.” MCP is the pipe. It is not a payment rail.
- Mint and burn Minting creates new stablecoin tokens against collateral. Burning destroys tokens when someone redeems. Issuers do that. If you are not the issuer, you receive and send existing tokens. Your ledger still records those movements. Calling a customer credit a “mint” hides that you did not create supply.
- Money movement map A money movement map is a diagram of how funds move through your product. Each arrow has a source and a destination. You draw the map before you write posting code, so every balance and every transfer has a place.
- Money transmission Money transmission is receiving customer funds and paying them out to someone else, or holding them in between. Many countries license that activity. An FBO or safeguarding structure is how teams avoid becoming the transmitter themselves. The ledger is how you prove you did not mix those funds with your own.
O
- OFAC OFAC is the US Office of Foreign Assets Control. It publishes sanctions lists. If a customer, a beneficiary, or a bank is on a list, you cannot send or hold that payment. The screen happens in your app or at the bank. The ledger records the transfer you were allowed to post, or the one you blocked and never sent.
- Omnibus account An omnibus account is a single bank account that holds funds for many customers. The bank sees one balance. Your ledger splits that balance into wallets, escrow, and other books. The split is your job. The bank will not do it for you.
- Open-loop payment system An open-loop payment system lets value leave your product on a public rail and arrive in an account you do not issue. Cards, ACH, and UPI are open-loop. A closed-loop system only moves value between wallets you issue. Most products start closed-loop and add an open-loop door for payouts and deposits.
- Optimistic locking Optimistic locking updates a row only if no one else changed it since you read it. The row carries a version. You write “set balance where version = 7.” If the version moved, the write fails and you retry. Pessimistic locking holds a lock for the whole read-compute-write. Optimistic assumes collisions are rare.
- Overdraft An overdraft lets a source balance go negative so a transaction can still post. The debit is larger than the credit on that balance. Use it when you mean to extend credit. Turn it off when a wallet or escrow must never go below zero.
P
- Parent transaction A parent transaction is the earlier record that a later record points to. Ledgers that do not edit history create a new row for each new state. The parent field is the link. Follow it and you get the lineage of a transfer.
- Payment rail A payment rail is the network that moves money between banks or wallets. ACH, RTP, FedNow, and wire are rails. A ledger is not a rail. The rail settles the bank money. The ledger records who owes whom while that happens.
- Precision Precision is the multiplier that turns a display amount into the currency’s smallest unit. Most fiat uses 100, so 10.50 becomes 1050. Bitcoin uses 100000000. Ledgers store that integer and do all math on it. They do not store floats, because floats drift.
Q
R
- Reconciliation Reconciliation is matching your ledger to an external record, such as a bank file or a card statement. The point is to find the posts that are missing, duplicated, or wrong. Matching rules decide what counts as the same payment.
- Reference A reference is the unique key you put on a transaction so the ledger can refuse a duplicate. If you send the same reference twice, the second request does not create a second movement. People often call this idempotency. The field name is reference.
- Refund A refund is a new transaction that sends value back along a path you already posted. It does not delete the original. The original stays. The refund points at it as its parent. Rails may also refund; your book still needs its own record.
- Reversal A reversal is taking back a payment you already originated, using the rail’s reversal process. In ACH, the ODFI can reverse a credit sent in error under Nacha rules and deadlines. It is not a customer refund and not a return. Your book still needs a new post the other way.
S
- Scheduled transaction A scheduled transaction is a transfer you create now to run at a time in the future. Until that time it is not applied. When the clock hits, it enters the same lifecycle as any other transaction and can apply, hold, or reject.
- Settlement Settlement is when value actually moves between the parties who owe it. Clearing is the matching of instructions. Finality is the point after which the payment will not be unwound. A ledger can record the obligation long before settlement, and must record the moment it happens.
- Source and destination Source and destination are the two sides of a ledger transfer. The source is where the value leaves. The destination is where it arrives. Every transaction has both. That pair is how a double-entry ledger records a movement without a one-sided post.
- Stablecoin A stablecoin is a token that aims to hold a stable value, usually one unit of a fiat currency. The chain moves the token. Your product still needs a ledger if you owe customers a balance, take a fee, or hold inventory. The wallet on-chain is not the customer book unless you have one customer and one address.
- Subsidiary ledger A subsidiary ledger is a detailed book that rolls up to one account in the general ledger. Customer wallets are the usual case: thousands of balances, one control account. The general ledger holds the total. The subsidiary holds who owns which part. They must add up.
- Sweep account A sweep account is a balance that you empty or top up on a rule: end of day, above a ceiling, below a floor. The sweep is a transfer you post, not a bank feature you get for free. If the bank also sweeps, you still record the movement or recon will break.
T
- Transaction A transaction is a movement of value from a source balance to a destination balance. It has an amount, a currency, a reference, and a status. Nothing else changes a balance. If the money moved in the product, the ledger has a transaction for it.
- Transaction lifecycle A transaction lifecycle is the set of statuses a transfer moves through from accept to finish. In Blnk those statuses are QUEUED, APPLIED, INFLIGHT, VOID, and REJECTED. Each new status is a new record that points at the last one. The old row stays.
V
W
- Wallet A wallet is a balance that belongs to a person or an organization in your product. It is not a bank account. It is a row in your ledger that you credit and debit as the customer uses the product. The bank account behind it is usually omnibus.
- Webhook A webhook is an HTTP callback the ledger sends when a transaction or other record changes. You do not poll for every status. You listen. A transaction hook is the same idea scoped to one event type. Deliveries retry. Your handler must be idempotent.
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