In blockchain terms, a block is a piece of code which includes information relating to a specific transaction of data on a distributed ledger (a blockchain). A new block of data can only be added to a blockchain with the consensus of a majority of the nodes which power the blockchain network. Once a block of data has been added to a blockchain, it cannot be deleted or altered – barring the discontinuation of all nodes making up the network across which the blockchain is recorded.
A block is made up of a block header and a hash tree. The header portion of a block is modeled after the genesis block of the blockchain which it is added to. The hash tree portion of a block contains data from previous blocks in the blockchain and new data pertaining to the new transaction which generated the block.