A terminology is a completely made-up slang term, or phrase. Sometimes the catch on, sometimes they don't
dude 1: "Oh snap! You wanna hear the new terminology I cooked up last night?"
dude 2: "Uh-oh"
dude 1: "check this out: I'm gonna call something cool that happens a 'seismic anomaly'"
dude 2: "okay, that's a terminology I can get into!"