Number of blocks found since the block a specific transaction was included in.
Each block has a set of transactions included in it, which the miners build on-top of as more blocks are confirmed. With each additional confirmation the more secure the transaction becomes.
With normal network activity it's possible for 1 block to be orphaned, sometimes 2, but any more than 3 confirmation is very secure.
Due to differing implementation and network hash-power, different coins require different amounts of confirmations to be considered secure by wallets and services.
Each confirmation adds another level of security, it's important to use the appropriate level of confirmations depending on the blockchain, and value of transaction
* All terms and definitions may update as the Cryptionary improves.