The purpose of this chapter is firstly to provide a definition of the aircraft, through an evaluation that takes into account the indications deriving from the analysis of the most relevant provisions of the 1944 Chicago Convention and the ICAO International Standards and Recommended Practices, of those emerging from the recent European derivative law instruments on aeronautical safety, as well as from some significant definitions existing in the legal system of some ICAO member countries. The definition in ICAO Annex 7 Aircraft Nationality and Registration Marks is examined by considering the various debates it has fuelled since its introduction, mostly in relation to the new frontiers opened up by technological progress. With particular reference to suborbital vehicles, and to the imminent employment of such vehicles for the transport of people and/or payloads in suborbital flight at super or ultrasonic speeds, attention is dedicated to their non-immediate inclusion in the definition of aircraft, with all the consequences that this entails in terms of applicable legal framework. Lastly, the aircraft is examined as a product, emphasising the importance of certification processes provided for by International Standards and Recommended Practices contained in Annex 8 Airworthiness of Aircraft for the safety of air navigation.