What is a block
A block is a structural unit of data in a blockchain that contains a set of verified transactions. Blocks are linked together to form a chain called a blockchain.
Each block references the previous block using a cryptographic hash. Because every block depends on the previous one, changing data in an earlier block would alter all subsequent blocks in the chain. This is why blockchain data is considered immutable.
What a block contains
A typical block in a blockchain includes several key components:
- a list of transactions
- the hash of the previous block
- a timestamp
- a nonce value
- the block's own cryptographic hash
These elements allow the network to verify the integrity of the blockchain and prevent modification of previously confirmed blocks.
How a block is created
The process of creating a block typically works as follows:
- users broadcast transactions to the network
- transactions are temporarily stored in a mempool
- transactions are grouped into a candidate block
- a miner or validator finds a valid solution for the next block
- the network verifies the block
- the block is added to the blockchain
In blockchains using Proof of Work, a miner creates the block by finding a valid hash.
Why blocks are important
Blocks perform several essential functions in blockchain networks:
- storing transactions
- securing the network
- preserving the transaction history
Because blocks are linked using cryptographic hashes, blockchain data becomes extremely difficult to alter.
Related terms
FAQ
What is a block in blockchain?
A block is a collection of transactions that is verified and added to the blockchain ledger.
How many transactions can a block contain?
The number depends on the specific blockchain and its block size limits.
Can a block be modified after it is added to the blockchain?
Practically no. Changing a block would require altering every subsequent block in the chain.