Block (Crypto)

A block is the fundamental data structure of a blockchain — a batch of validated transactions grouped together, timestamped, and cryptographically linked to the previous block. Each block contains: a block header (with metadata including the previous block's hash, timestamp, nonce, and Merkle root), a list of transactions, and a unique hash that identifies the block.

Block parameters vary significantly across blockchains. Bitcoin produces blocks approximately every 10 minutes, with a maximum size of 4MB (post-SegWit). Ethereum generates blocks every ~12 seconds. Solana targets 400-millisecond block times. These parameters directly affect network throughput, finality time, and decentralization — faster blocks generally require more powerful validators, potentially reducing who can participate.

The block creation process differs by consensus mechanism. In Proof of Work, miners compete to find a valid hash by adjusting the nonce — the first to succeed earns the block reward plus transaction fees. In Proof of Stake, validators are algorithmically selected to propose blocks based on their stake. The sequential linking of blocks through cryptographic hashes creates immutability: altering any historical block would change its hash, breaking the chain and requiring recalculation of every subsequent block — computationally infeasible for established blockchains.