Block

In Bitcoin, a block is simply a group of transactions that are recorded together. Each block is connected to the one before it, forming a continuous chain called the blockchain. Typically, a Bitcoin block contains about 2,700 individual transactions and can be up to 4 megabytes in size.

For a block to become part of the blockchain, it must meet the requirements of Bitcoin’s Proof-of-Work system. This involves finding a special hash that proves work has been done to create the block. Every block also includes the hash of the block before it, effectively “linking” them together. Because each hash depends on the contents of both the current and previous blocks, even the slightest change in one block will alter all subsequent hashes, making tampering easy to detect and practically impossible.

For instance, if someone tried to modify a transaction in block #400, the hash for that block would change, breaking the chain for block #401 and every block after. Thanks to this setup, once a block is added to the Bitcoin blockchain, its data—and all transactions it contains—are permanently locked in place.