Quick Definition

A block is a structured data unit that batches validated Bitcoin transactions and links to the previous block via its hash, forming one step in the ever-growing blockchain ledger.

Key Takeaways

• Timestamp and Height: Records when the block was mined and its sequential position (block height) for clear chronological ordering.
• Block Reward: Awards newly minted bitcoin (subsidy, halved every ~4 years) plus transaction fees to the miner, incentivizing network security through proof-of-work.

In-Depth Explanation

When Bitcoin users broadcast transactions, they enter a node’s mempool, a pool of unconfirmed transactions awaiting confirmation. Miners select transactions from their mempool, prioritizing higher fees, and verify their validity (no double-spends, correct signatures, sufficient fees). These transactions are assembled into a candidate block, limited to 4 million weight units (1 MB pre-SegWit) to manage network throughput.

To add the block to the blockchain, a miner must solve a proof-of-work puzzle: finding a nonce (and sometimes adjusting other header fields) that makes the block header’s SHA-256 hash fall below a network-wide difficulty target. The header includes the previous block’s hash, linking the chain, and the Merkle root, summarizing transactions. This energy-intensive process, adjusted every 2016 blocks (~2 weeks) to maintain a ~10-minute block interval, ensures that rewriting the blockchain’s history is computationally and economically prohibitive.

Once a miner finds a valid hash, they broadcast the new block. Full nodes independently verify the proof-of-work, every transaction’s validity, and the block’s structure (e.g., size, Merkle root accuracy). If all checks pass, nodes append the block to their blockchain copy, update their UTXO set (unspent transaction outputs), and propagate the block. Rarely, simultaneous block discoveries cause temporary forks, but nodes follow the longest chain with the most cumulative work.

This process repeats roughly every 10 minutes, creating an immutable, chronological ledger secured by cumulative proof-of-work. The block subsidy (e.g., 3.125 BTC in 2025, post-2024 halving) and transaction fees reward miners, ensuring Bitcoin’s decentralized security without a central authority.