DEFINITION
A batch of transactions bundled together, verified, and added to the blockchain roughly every ten minutes.
Miners collect pending transactions from the mempool, pack them into a candidate block, and race to find the proof-of-work solution that lets them add it to the chain. The winner broadcasts the block and collects the block reward plus fees.
Each block contains a cryptographic fingerprint of the previous one — that linkage is what makes rewriting history effectively impossible and gives the "chain" its name.