Block
mining · beginner
A batch of transactions plus a header, added to the chain roughly every ten minutes when a miner finds a valid hash.
A block is the unit by which Bitcoin's history advances. Each contains an 80-byte header (previous block hash, merkle root, timestamp, difficulty target, nonce) and a list of transactions. Miners search for a nonce that makes the block header hash fall below the current target; whoever finds one broadcasts the block and the rest of the network verifies it.
The "previous block hash" field is what makes the chain a chain — every block commits to the one before it, so changing an old block would require redoing every block after it. Combined with proof-of-work, this is what makes Bitcoin's history effectively immutable: the deeper a transaction sits, the more energy an attacker would have to expend to revise it.
Blocks are produced approximately every 10 minutes; the difficulty target adjusts every 2,016 blocks (~2 weeks) to maintain that pace as hash rate moves. A block's contents are capped at ~4 million weight units, a metric that replaced the older 1-MB raw-size limit when SegWit activated.