aircraft = any machine capable of flight, including airplanes, helicopters, hot air balloons, blimps and dirigibles - The aviation museum has examples of almost ever kind of aircraft ever used. Airplane = aircraft with wings. Commonly used in everyday speech. - Most airplanes have one set of two wings. Biplanes have two sets of wings, and triplanes haves three sets. - A 747 is a really big airplane. Aeroplane = airplane in British English - My spell-checker is set for American English, and so "aeroplane" is always marked as misspelled. Plane = short for airplane. Even more commonly used than "airplane." - I like to go to the airport to watch the planes land and take off. - (while leaving the airport) Oh, no! I think I left my sweater on the plane. - I got a good price on my plane ticket.

I agree with Evelyn and Jerry's definitions. Plane, airplane and aeroplane refer to the same object. But whilst all planes are aircraft, not all aircraft are planes. And just a small add-on from a British English perspective: The short form "plane" is by no means just an American usage. Because our version of the American word "airplane" still has three rather than two syllables - "aeroplane" as opposed to the simplified "airplane" - we are even more likely to say "plane" in everyday usage.

They're synonyms but we haven't used the spelling 'aeroplane' in the US since the 1920s or so.