What Is the Significance of the “Nonce” in the Process of Finding a Valid Block Solution?

The "nonce" (number only used once) is a variable field in the block header that miners iterate through to change the resulting hash of the block header. The goal is to find a nonce value that, when combined with the rest of the block data, produces a hash that is below the network's target difficulty.

Finding the correct nonce is the final step in the proof-of-work process and signifies a valid block solution.