The nonce is a field in the block header.
I call it "the mining field".
What's it used for?
Miners adjust the nonce when they're trying to get the hash of their block headers below the target. (i.e. Get a valid Block ID for their candidate block.)
All of the other fields in the block header are predetermined, so the nonce is what allows you to create different hash results. Block 404,104
Why is it called a "nonce"?
Just one of those terms used in cryptography, like Alice, Bob, and whatnot.
In cryptography, a nonce is an arbitrary number that may only be used once.
I know, I prefer "the mining field" too.