A fixed-size fingerprint produced by running data through a hash function.
Hash meaning
Bitcoin uses cryptographic hash functions to turn arbitrary data into fixed-size outputs. Small input changes produce unpredictable output changes. A good cryptographic hash is easy to calculate and impractical to reverse or deliberately collide.
How it works in Bitcoin
Hashes identify transactions and blocks, connect headers, build Merkle trees, and power proof of work. Bitcoin commonly applies SHA-256 twice in these protocol structures, while other encodings use additional functions.
A simple example
Changing one byte in a transaction changes its transaction hash. That change also alters the Merkle commitment in any block containing it.
Why Hash matters
Network terms explain how independently operated software shares, validates and orders information. They help separate a convenient data viewer from the rules a node actually enforces.
What people often get wrong
Hashing is not encryption. There is no decryption key that restores arbitrary original data from a hash.
Quick checks before using the term
- Which actor performs this job?
- Is the behavior a consensus rule or an implementation choice?
- What evidence can a node verify independently?
Apply these questions to the concrete example above. A precise answer is more useful than repeating the definition without naming the actor, rule or failure path.
Check the primary reference
This entry is written in plain English, but the technical source is available when you need exact protocol detail.
Bitcoin Developer Guide: Block Chain ↗