Library · Crypto, word by word · The basics
What is a block?
A block is a batch of transactions added to a blockchain together. Each block contains a hash (a fingerprint) of the previous block, which is what links the blocks into a chain. Ethereum, for example, adds a block every 12 seconds.
What it means for you
Your payment is not recorded when you press send, only when it is included in a block, and each later block makes it harder to reverse. Changing any past block would change every hash after it, so the network rejects the altered history.
How it works
On Ethereum, one validator chosen at random proposes a block in each 12-second slot. The block holds the parent's hash, a list of transactions and a state root, a hash of the whole state after those transactions; every client re-executes them and rejects the block if its result does not match. A gas limit caps how much computation one block can demand, so blocks cannot grow arbitrarily large. On Bitcoin, an 80-byte block header commits to all the block's transactions through a merkle root, and that header is what miners hash for proof of work.
Sources: ethereum.org: Blocks, bitcoin.org: Vocabulary, Bitcoin developer glossary · checked 4 October 2026
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