A Bitcoin block is a group of Bitcoin transactions that have been verified and added to the Bitcoin blockchain. All Bitcoin transactions must be verified and recorded in a block before they can be considered valid. Each block is linked to the previous block, forming a chain of blocks that show every transaction made in the history of Bitcoin. Each block also contains a cryptographic hash of the previous block, creating a digital signature that ensures the data in the block cannot be changed and that all blocks are securely linked together. A new block is created every 10 minutes on average, and all miners compete to be the first to solve a mathematical problem to add the next block to the chain.