It is a pedantic but meaningful distinction that I'd only point out on a sorta geeky site like this, but actually, FL180 (or, flight level one-eight-zero) is the altitude at which an altimeter will read 18,000 feet if it's set to assume that the barometric pressure is 1013 hPa (29.92 inHg). Above a certain transition altitude, aircraft switch their altimeters from reading altitude in the local pressure to this "standard" pressure. This is because above that altitude and safely away from terrain, it's no longer important to know precisely how high you are, but it _is_ important to know what altitude you are relative to all the other aircraft nearby.