Transactions Confirmation

A transaction is said to be confirmed when either:

• The block in which is included is considered final by the consensus algorithm(this happens is some PoS blockchains)
• The block in which is included has enough child blocks that it is considered final by convention (ex. 6 for BTC)

With the "number of confirmation" usually one refers to the number of child blocks of the block in which the transaction is included

Sources:

https://fintechmagazine.com/financial-services-finserv/140-blockchain-and-crypto-words-ultimate-z-glossary