A blockchain block is a group of transactions on a blockchain. When a block is “confirmed” it becomes a part of the immutable ledger known as the blockchain and all the transactions in it are confirmed with it. Once a block is confirmed, all the transactions in it cannot be changed or charged back. Depending on the blockchain, blocks hold can hold differing numbers of transactions and are confirmed at different rates. Bitcoin blocks usually contain between two and three thousand transactions and take roughly 10 minutes to be confirmed.