💡 The Plain-English Definition
A block is a batch of confirmed Bitcoin transactions, packaged together and permanently added to the blockchain approximately every ten minutes. Each block links cryptographically to the one before it, forming the chain that gives Bitcoin its immutability.
🤔 But Why Though?
Bitcoin transactions don’t confirm individually. Instead, they wait in the mempool (Bitcoin’s waiting room for unconfirmed transactions) until a miner selects them, packages them into a block, and solves the proof-of-work puzzle (the energy-intensive computational task required to add a valid block). Once a miner finds a valid solution, they broadcast the block to the network. Other nodes verify it and add it to their copy of the blockchain.
Every block has two main components. The header — just 80 bytes — contains the version (which protocol rules apply), the previous block hash (the cryptographic fingerprint linking this block to the last one), the merkle root (a compact mathematical summary of all transactions in this block), the timestamp, the difficulty target, and the nonce (the number miners iterated through to find a valid solution). The body contains the actual transaction data — typically between one thousand and several thousand transactions — with the coinbase transaction (the miner’s reward payment) always appearing first.
The linking mechanism is what creates immutability. Each block’s header contains the hash (a unique cryptographic fingerprint — a fixed-length mathematical summary) of the previous block’s header. Change anything in block 500 and its hash changes, which invalidates the reference stored in block 501, which invalidates block 502, and so on to the current tip. Repairing the chain would require redoing the proof-of-work for every block from 500 forward — at today’s mining speeds, a task that would require 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 happened. Each page begins with a unique fingerprint of the previous page — so if anyone tries to alter an old page, all subsequent pages fail their verification. The further back in the book the alteration is, the more pages that would need to be re-done to hide the tampering. Eventually, redoing them all would take longer than the town’s entire history. That’s what blocks chained 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 containing your transaction. Each additional block makes it exponentially harder to reverse. Six confirmations is the traditional standard for large or irreversible transfers, because by that point the computational cost of reversal is essentially prohibitive.