Crypto confirmations are covered in full in the entry Transaction Confirmations (TCs), which explains how they are counted, how many different services require and why. This page gives a short definition and explains the plural form.
The number of confirmations of a transaction is the number of blocks that have been added to the blockchain since, and including, the block that recorded it. A transaction in the latest block has one confirmation; after five more blocks it has six. Because each new block makes it harder for an attacker to rewrite history, exchanges, merchants and wallets wait for a set number of confirmations before treating a payment as final. The number depends on the network's block time and security and on the amount at stake: Bitcoin deposits are often credited after a few confirmations, while smaller proof-of-work chains may require hundreds. Networks with formal finality, such as Ethereum, also use finalized checkpoints, described under probabilistic finality.
"Confirmations" in the plural usually refers to this count, while a single confirmation describes the event of a transaction being included in a block.