Block

A block is the basic unit in which a blockchain records data: a bundle of transactions plus a header that links it to the block before it. When a miner or validator produces a block, the network checks that every transaction inside it is valid and that the block follows the protocol's rules, then each node appends it to its own copy of the chain. Because the header contains a hash of the previous block, editing an old block would change every hash after it — which is what makes recorded history hard to rewrite. Blocks have size or gas limits, so when demand exceeds capacity, transactions compete on fees for the remaining space.