block

A block is a bundle of recent transactions, packaged together and added to a blockchain as a single batch. Rather than writing every transaction to the shared ledger one at a time, a network collects a few hundred or a few thousand of them, wraps them up, and appends the whole parcel at once. Think of it as a single page in a communal account book: each page holds many entries, the pages are filled in order, and once a page is full it is glued permanently into the binding.

What makes a block more than just a list is how it is sealed and chained. Every block carries a compact summary called a header, which includes a cryptographic fingerprint (a hash) of the block before it. Because that fingerprint changes completely if even one character of the earlier block is altered, each block effectively locks the entire history that came before it. Tamper with an old page and every later page's seal stops matching, so the whole network instantly sees that something is wrong.

Blocks are why a blockchain can be both append-only and trustworthy without any central record-keeper. New blocks arrive at a roughly steady pace set by the network's rules, and once enough later blocks pile on top, a block becomes practically impossible to rewrite. This steady, chained, batch-by-batch growth is the literal shape behind the name blockchain: a chain of blocks, each one vouching for all the ones before it.