A batch of transactions permanently recorded on a blockchain. Each block links to the previous one, forming an unbreakable chain.
A block is the fundamental data unit of a blockchain — a collection of validated transactions bundled together and permanently added to the chain. Each block contains: a header (with metadata including the previous block's hash, timestamp, and nonce), a body (the actual transaction data), and a hash (a unique cryptographic fingerprint of the block's contents). The inclusion of the previous block's hash in each new block creates the 'chain' in blockchain — altering any historical block would change its hash, invalidating every subsequent block. Block sizes and times vary by network: Bitcoin blocks are ~1-4MB created every ~10 minutes, Ethereum blocks are created every ~12 seconds, and Solana produces blocks every ~400 milliseconds. The block production rate directly impacts transaction throughput and finality speed.
A block is a bundle of validated transactions permanently recorded on the blockchain, chained to the previous block through cryptographic hashes — creating an append-only ledger that cannot be altered without re-doing the work for every subsequent block. Block parameters vary significantly across chains: Bitcoin produces blocks every ~10 minutes containing ~2,500 transactions (1MB limit), while Solana produces blocks every 400 milliseconds with thousands of transactions each. Block size and frequency directly impact a chain's throughput, fees, and decentralization trade-offs — larger, faster blocks require more powerful hardware to validate, potentially reducing the number of people who can run nodes. The 'block space' (capacity available in each block) is a scarce resource that drives fee markets: when demand for block space exceeds supply, users bid up gas prices.
Bitcoin block #800,000 was mined in July 2023 — it contained 3,721 transactions, was 1.56MB in size, and the miner received 6.25 BTC (~$190,000) as a block reward plus transaction fees for creating it.
Block size reflects a design trade-off: larger blocks enable more transactions per second but require more bandwidth, storage, and processing power to validate — making it harder for regular users to run full nodes. Bitcoin chose small blocks for maximum decentralization; chains like Solana chose large blocks for maximum throughput.