The actual "540" is a jumping inward crescent. Anything besides that and it's going to be mentioned, like "540 hook", or something. A 540 roundhouse is REALLY hard to do and make it an actual roundhouse given the mechanics for the turn.
Tornado kick: jump spinning inward crescent. Same mechanics as a 360 roundhouse, where the non-jumping leg is brought up first (completely chambered) about 180 degrees before the actual kick is thrown up (which should be in the same direction as you started), landing on the non-kicking leg. It's a pretty common kick, but people don't always recognize the name because it comes from American Karate. It's the precursor kick to a standard 540 because you have to get the non-kicking leg up perfectly and be able to maintain balance perfectly before you can think about doing 540s very well (being that the non-kicking leg has to be thrown HIGH and then gotten out of the way so you can land on the kicking leg!).