Proof of Work (PoW)

Definition

Proof of work is a mechanism that makes producing an acceptable result computationally costly while allowing others to check that result comparatively quickly. Bitcoin uses it by requiring a candidate block header's hash to meet a numerical target. Miners perform repeated trials, and finding a valid result provides evidence of an expected amount of hashing effort rather than a record of every calculation attempted. Nodes compare accumulated work when choosing among valid blockchain histories. This makes rewriting history require additional competitive work, but proof of work does not replace transaction validation or establish that any external information is true. It also does not mean miners are solving useful scientific problems; Bitcoin's work is specifically tied to its block-selection process. Difficulty adjusts the expected effort, while the random nature of hashing means individual miners can experience long or short intervals between successes.