What is a block in a blockchain?
Answer
A block is the fundamental unit of data in a blockchain. Each block contains: (1) Block header — metadata including the block number (height), timestamp, previous block's hash (creating the chain), Merkle root (cryptographic summary of all transactions in this block), and nonce (used in Proof of Work); (2) Transactions — the batch of validated transactions (transfers, smart contract calls) included in the block; (3) Block hash — the cryptographic fingerprint of this block's header. The chain of hashes is the security mechanism: changing data in any block changes its hash, breaking the link to all subsequent blocks. This makes retroactive tampering computationally detectable. Bitcoin blocks average 1MB every 10 minutes; Ethereum blocks are limited by gas (computation) rather than size, with blocks produced every ~12 seconds.