In computing, a block is a sequence of bytes or bits, usually containing records, and having a maximum length (block size). File systems and databases are based on blocks, which store and retrieve blocks of data.
The Bitcoin Whitepaper by Satoshi Nakamoto mentions the block in the context of its timestamping protocol. A block is a group of items to be timestamped and widely published with its hash. The timestamp proves that the data must have existed at the time in order to produce the hash. Each timestamp includes the previous block’s hash in its hash, thus forming a chain, with each additional timestamp reinforcing the ones before it.
Blocks have to be valid under the cryptocurrency protocol, nodes accept valid blocks by adding them to the chain of blocks, and work on extending them by mining, and reject invalid blocks by ignoring them.
The block items are transactions, which are hashed in a Merkle Tree, with only the root included in the block’s hash. This proves the order of transactions under a cryptocurrency protocol to prevent double-spend attacks.