Mono is short for monofilament, Nylon is the fiber that the line is made of. mono just means it’s a single strand line. Most recreational fisherman call monofilament “Nylon” probly cause in the early days this is what monofilament was marketed as.
In our shop nylon as a fiber is also used in rope for anchoring and clothing etc.
It’s a funny one, most selling the line call it mono or monofilament and very one buying it calls it nylon.