A set of transactions confirmed on the blockchain. Each block builds on the blocks before it, creating a chain of blocks back to the genesis (first) block.
This chain of blocks is known as a blockchain.
When a miner confirms a set of transactions it is known as a block.
As soon as a miner finds a block, it is broadcast to the other miners. At this time the other miners validate the block, and they begin searching for the solution to the next block.
When transaction is in a confirmed block it's confirmation count increases by 1, with each block found after adding another confirmation to it.
The more confirmations a transaction has, the more secure it is. On most blockchains payments can be considered sufficiently secure after 0 to 10 confirmations - depending on the value of the transaction, the hashrate, and other properties of each blockchain.
* All terms and definitions may update as the Cryptionary improves.