Definition
Proof of Work (PoW) is the mechanism that secures Bitcoin and prevents double-spending. Miners must find a hash of the block header that is numerically below a target value determined by the current difficulty. This requires enormous amounts of computation (and therefore energy), making it extremely expensive to attack the network.
The beauty of PoW is that while finding the solution is computationally expensive, verifying it is trivial. Any node can instantly verify that a block hash meets the difficulty target. This asymmetry is what makes Bitcoin secure and trustless.
In Simple Terms
Bitcoin's security mechanism requiring miners to expend energy to validate transactions and create blocks.
The consensus mechanism used by Bitcoin where miners must expend computational energy to find a valid block hash, proving they performed work to secure the network.