💡 The Plain-English Definition
A block is a batch of confirmed Bitcoin transactions, packaged together and permanently added to the blockchain about every ten minutes. Each block is cryptographically linked to the one before it, forming the chain that makes Bitcoin’s past practically impossible to rewrite.
🤔 But Why Though?
Bitcoin transactions don’t confirm one at a time. They wait in the mempool — the waiting room where unconfirmed transactions sit — until a miner picks them up and packages them into a block. To make that block count, the miner has to solve the proof-of-work puzzle, the costly computation required to add a valid block. Once a miner finds a valid solution, it broadcasts the block to the network. Other nodes — computers each keeping their own full copy of the ledger — check it and add it to their copy of the blockchain.
Every block has two parts, a header and a body. The header is tiny — just 80 bytes — but it packs in a lot: the version (which protocol rules apply), the previous block’s hash (the fingerprint linking this block to the last), the merkle root (a compact summary of every transaction in the block), the timestamp, the difficulty target (how hard this block was to find), and the nonce (the number the miner changed over and over to find a valid solution). The body holds the actual transactions — usually one thousand to several thousand of them — with the coinbase transaction, the miner’s reward payment, always first.
That linking is what makes the chain so hard to change. Each block’s header carries the hash of the block before it, so the blocks are tied together in order. Change anything in block 500 and its hash changes. That breaks the reference stored in block 501, which breaks block 502, and so on up to the newest block. Repairing the chain would mean redoing the proof-of-work for every block from 500 onward — at today’s mining speeds, more computing power than exists on Earth.
🌍 The Real-World Analogy
Imagine a town’s official record book where a new page is added every ten minutes, summarising everything that just happened. Each page begins with a unique fingerprint of the page before it. So if someone alters an old page, every later page fails its check. The further back the change, the more pages you’d have to redo to hide it. Eventually, redoing them all would take longer than the town has existed. That’s what blocks chained together by hashes achieve.
⚡ So What?
When you hear that a Bitcoin transaction has “3 confirmations,” it means three blocks have been added to the chain since the block that included your transaction. Each further block makes reversing it much harder — the difficulty climbs steeply with each one. Six confirmations is the traditional bar for large or irreversible transfers, because by that point the cost of reversing it is far too high to be worth anyone’s while.
