The nonce is a field in the block header.
I call it “the mining field”.
Nonce is an abbreviation of number used once.
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 Hash for their candidate block.)
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.