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A block in cryptocurrency is the fundamental unit of a blockchain: a data container that groups together a set of validated transactions, links to the previous block through a cryptographic hash, and carries metadata including a timestamp and the answer to a computational puzzle. When miners or validators add a new block to the chain, the transactions inside become a permanent part of the network's shared ledger.
Think of it as a sealed page in a public ledger: every transaction recorded on that page is final, and the page is physically attached to the one before it.
A block has two main components. The block header contains summary information about the block itself, including the hash of the previous block, a Merkle root that is a single hash representing all transactions in the block, a timestamp, the current network difficulty target, and the nonce used in mining. The block body contains the list of actual transactions, starting with the coinbase transaction that awards new cryptocurrency to the miner.
The hash of the previous block in each block header is what creates the "chain" in blockchain. Change any data in a historical block and its hash changes, breaking the link to every block that came after it. Altering history requires recomputing every subsequent block, which is why the chain becomes progressively more immutable over time.
The amount of transaction data a block can hold determines how many transactions per second the network can process. Bitcoin originally had a 1 MB block size limit, which created congestion when usage grew. The 2017 SegWit upgrade and the 2021 Taproot upgrade increased effective capacity without formally changing the block size. Larger blocks mean more transactions per block but also larger data storage requirements for every node running the network.
Different blockchains make different trade-offs. Ethereum measures block capacity in gas units rather than bytes. Solana uses parallel processing to increase throughput without simply increasing block size.
Block time is the average interval between successive blocks being added to the chain. Bitcoin targets 10 minutes per block, producing roughly 6 blocks per hour. The network automatically adjusts mining difficulty every 2,016 blocks (approximately every two weeks) to maintain that 10-minute average as total mining power rises or falls.
Ethereum's block time is approximately 12 seconds since the shift to Proof of Stake in September 2022. Faster block times mean faster transaction confirmation but also more frequent small blocks, which can create coordination problems between validators.
The first block in any blockchain is called the genesis block. It has no previous block to reference, so its "previous hash" field contains zeros. Bitcoin's genesis block was mined by Satoshi Nakamoto on January 3, 2009. Embedded in its coinbase transaction is the text "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks," a timestamp and a philosophical statement about why Bitcoin was created.
Sources:
https://en.wikipedia.org/wiki/Blockchain
https://www.theblock.co/learn/245697/what-are-blocks-in-a-blockchain
https://www.gemini.com/cryptopedia/what-is-block-in-blockchain-bitcoin-block-size
https://learnmeabitcoin.com/beginners/guide/blocks/
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