How Does the ‘Nonce’ Relate to the Proof-of-Work Puzzle?

The 'nonce' is a number used only once (Number Once). It is a variable that miners repeatedly change and input into the block header hash function.

The goal is to find a nonce that, when hashed with the rest of the block data, produces a hash value that meets the network's difficulty target (e.g. starts with a certain number of zeros). This trial-and-error process is the computational work required for PoW.

Finding the correct nonce proves the work was done.