Block
A batch of transactions recorded together, plus a header that links it to the block before it.
The header carries the hash of the previous block, a Merkle root committing to every transaction inside, a timestamp, and consensus data such as the proof-of-work nonce or the proposer's signature. How much a block can hold is capped by a size limit, a gas limit, or a compute budget, which is what creates competition for space and therefore fees. Producing a valid block is the job of a miner in proof of work or a validator in proof of stake, and every other node re-checks the result before accepting it. A reader meets blocks in block explorers, where each one lists its height, timestamp, transaction count, and producer.
In practice
A Bitcoin block header commits to all of its transactions through a single Merkle root, so changing any one transaction changes the header and invalidates the block.
The common misunderstanding
A block is not a fixed number of transactions; it is bounded by size or gas, so one block may hold a few heavy transactions and another many light ones.