What Is A Transaction Confirmation?

In cryptocurrencies, once a transaction is part of a block and that block becomes part of the blockchain by network consensus, then we say that "the trascaction has one confirmation".

When the next block is mined and integrated to the blockchain, the transaction has two confirmations, etc., etc.

Six or more confirmations is considered sufficient proof that a transaction cannot be reversed, but some services may require more or sometimes fewer confirmations.