block

A block is the fundamental unit of the Bitcoin blockchain. Each block has two main components: a header and a transaction list. The header contains the hash of the previous block, a Merkle root that summarizes all included transactions, a timestamp, the current difficulty target, and a field called the nonce. Linking each block to its predecessor via the previous block's hash is what forms the chain: altering any historical block would break the chain from that point forward and require redoing all subsequent proof-of-work.

To produce a valid block, a miner must find a nonce value such that the SHA-256 hash of the block header falls below the current difficulty target. This requires vast computational effort and cannot be shortcut or predicted. The Bitcoin protocol adjusts the difficulty every 2,016 blocks (approximately every two weeks) to ensure new blocks are found roughly every 10 minutes on average, regardless of how much computing power is active on the network.

Each block added on top of an existing block represents one confirmation for the transactions it contains. A transaction in the most recent block has one confirmation; after the next block is found, it has two, and so on. More confirmations mean more accumulated proof-of-work would need to be undone to reverse the transaction. For small everyday transactions, one or two confirmations are usually sufficient. For large transfers, six or more is the common standard.