Block

In simple terms

A block is a page in the blockchain's ledger. Transactions are grouped onto a page, the page is filled and sealed, and the next page begins. Each page references the one before it, which is what makes it a chain.

Definition

A batch of transactions recorded together on a blockchain, linked to the block before it.

In depth

A block is a data structure containing a header and an ordered list of transactions. The header holds the hash of the previous block, a Merkle root summarising every transaction inside, a timestamp, and consensus-specific fields such as a nonce and difficulty target on proof-of-work chains. Because the header commits to the previous block's hash, altering any historical transaction changes that block's hash and invalidates every block after it, which is the mechanism behind a blockchain's immutability rather than a policy anyone enforces.

How does Block work?

Transactions broadcast by users collect in a waiting area called the mempool. A miner or validator selects a set of them, usually favouring higher fees, assembles them into a block, and produces the proof the network requires — solving a hash puzzle under proof of work, or being selected and signing under proof of stake. The completed block is broadcast, other nodes verify every transaction and the proof independently, and each appends it to their own copy of the chain. The next block then references this one by hash.

An example

A Bitcoin block arrives roughly every ten minutes on average and holds on the order of a few thousand transactions. If an illustrative 3,000 transactions are waiting and a block carries 2,000, the remaining 1,000 stay in the mempool and compete for the next one. Intervals vary because block production is probabilistic, so two blocks can arrive a minute apart and the next take half an hour.

Figures are illustrative only.

What beginners get wrong

• Treating the average block time as a schedule. Ten minutes is a long-run average, and any individual gap can be far shorter or far longer.
• Assuming a transaction is in a block the moment it is sent. Until it is included it is only in the mempool, where it can sit for a long time or be dropped.
• Thinking a block stores account balances. It stores transactions, and balances are derived by replaying them.

Related terms

• Blockchain
• Transaction
• Confirmation
• Hash
• Mining
• Immutability

Part of

How does a blockchain transaction actually work? — the subject page for blockchain mechanics, with all 15 of its definitions in one place.