Block
A batch of transactions bundled together and permanently added to the blockchain.
A block is the fundamental unit of a blockchain. It contains a set of validated transactions, a timestamp, a reference to the previous block (its hash), and metadata such as the miner or validator's address.
Each new block extends the chain and makes all prior transactions harder to reverse — the more blocks built on top of a transaction, the more 'confirmations' it has and the more secure it is. Bitcoin produces a block roughly every 10 minutes; Ethereum targets ~12 seconds.
Block size (the number of transactions per block) and block time (how frequently blocks are produced) are key parameters that affect a network's throughput and fee levels.