Glossary
Proof of Work (PoW)
A consensus mechanism in which block producers compete to find a hash below a target, spending real-world energy as the cost of writing to the chain.
Proof of work asks miners to find a nonce that makes a block's hash fall below a network-wide difficulty target. The only known way to do this is to try hashes — billions per second on dedicated hardware — until one works.
The energy spent is the security: rewriting history requires re-doing all that work faster than the honest network keeps adding to the tip. Bitcoin still uses PoW; many other chains have moved away from it on environmental or efficiency grounds.