oohhh a question i can answer seeing as i'm currently in the air force training to become an ATC
Yes 18,000MSL and up in the US at least is referred to as flight levels (FL180) and at or above FL180 altimeters are set to 29.92 under FL180 its set to the local setting
Also FL180 up to and including FL600 is Class A airspace and is IFR only (see http://www.asy.faa.gov/safety_products/airspaceclass.htm) and transponders are only required by aircraft with electrical systems. Since you could technically be an AC i guess without an electrical system you would be ok
