An experimental aircraft is an aircraft that has not yet been fully proven in flight. Often, this implies that new aerospace technologies are being tested on the aircraft, though the label is more broad.
The term "experimental aircraft" is also sometimes used to refer to aircraft flown with an "Experimental" category airworthiness certificate. In the U.S. this includes most homebuilt aircraft; many of which are based on conventional designs and hence are experimental only in name.
See also