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A set of transactions confirmed on the blockchain. Each block builds on the blocks before it.
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When a miner confirms a set of transactions it is known as a block.

As soon as one block is found, it is broadcasted to the network and propagates to the other miners. At this time the miners begin searching for the next block. After a transaction is included in a block it's confirmation count increase 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.