Definition
A Bitcoin block is the fundamental unit of the blockchain. Each block contains a header with metadata (version, previous block hash, merkle root, timestamp, difficulty target, and nonce) plus a list of validated transactions.
The maximum block size is effectively around 4 MB (with SegWit), though most blocks are smaller. Miners compete to find a valid hash for the next block, and the winner gets to add it to the chain and collect the block reward plus transaction fees.
In Simple Terms
A bundle of Bitcoin transactions added to the blockchain. Miners compete to create each new block.
A collection of Bitcoin transactions bundled together and added to the blockchain. Each block contains a block header, a list of transactions, and a reference to the previous block.