Block

Beyond simply storing data, a block acts as a verified, tamper-evident package: it bundles a batch of transactions together with metadata that proves where those transactions fit in the ledger's history. Every block contains two main parts: a header and a body. The header holds the timestamp, a difficulty target, a nonce used during mining, and two critical hashes, one linking back to the previous block and one summarizing all transactions inside the current block via a Merkle root. The body holds the actual transaction data itself.

Because each header references the hash of the block before it, altering a transaction buried deep in the chain would change that block's hash and break every link after it, forcing an attacker to redo the work for every subsequent block. This chaining is what gives a blockchain its resistance to tampering, rather than any single block being special on its own.

Block design varies by network. Bitcoin originally capped blocks at 1 megabyte; the 2017 SegWit upgrade replaced that limit with a 4 million weight-unit cap, giving a practical size closer to 1.5 to 2 megabytes, produced roughly every 10 minutes. Ethereum instead sizes blocks by total gas used, targeting a new block roughly every 12 seconds. Larger blocks fit more transactions but raise the storage and bandwidth burden on node operators, a tradeoff at the center of long-running debates over decentralization versus throughput.

Occasionally two miners solve a block at nearly the same time, producing a temporary fork. The network eventually settles on the longer chain, and the blocks left on the shorter branch become "orphaned" (or "uncled" on some chains) once they are no longer part of the accepted history.