Definition
When a transaction is included in a block, it has 1 confirmation. Each subsequent block added to the chain adds another confirmation. The more confirmations a transaction has, the more computationally expensive it would be to reverse it.
For small transactions, 1-3 confirmations are typically sufficient. For large amounts, 6 confirmations (approximately one hour) is the traditional standard. Exchanges and merchants set their own confirmation requirements based on risk tolerance.
In Simple Terms
The number of blocks mined after a transaction, indicating how secure and irreversible it is.
The number of blocks that have been added to the blockchain after a given transaction's block. More confirmations means higher confidence that the transaction is irreversible.