If an aircraft cannot fly above 300ft, is it an aircraft?
There is actually a legal Australian definition for aircraft, in Part 1 section 3 of the Civil Aviation Act:


  aircraft means any machine or craft that can derive support in the atmosphere from the reactions of the air, other than the reactions of the air against the earth’s surface.


So those drones you can buy from the store are aircraft (and if you knock one out of the sky because it annoys you, you are technically bringing down an aircraft and could be punished). However the latter half of that definition suggests hovercrafts are not aircraft. 

Also, the minimum height over populated areas in Australia is 1000ft, and 500ft over non-populated areas (CAR157)