Confirmation (block confirmations)

A confirmation means a transaction has been included in a block. When it first lands in a block it has one confirmation; each additional block added on top gives it one more. The more confirmations, the harder it is to reverse, because an attacker would need to rebuild all of those blocks faster than the rest of the network.

Exchanges and merchants set their own requirements before crediting a deposit. For Bitcoin, many wait for between one and six confirmations, with six (roughly an hour) a long-standing convention for large amounts. Faster networks may require dozens of blocks, but because their blocks are quicker the total wait can be shorter. On proof-of-stake chains such as Ethereum, services may instead wait for "finality", a point after which reversal would require destroying a large amount of staked ETH.

If a deposit is slow, check the TXID on a block explorer. A transaction with a low fee can sit unconfirmed in the mempool for a long time when the network is busy.