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 usually used to refer to aircraft flown with an experimental certificate. In the United States, this includes most homebuilt aircraft; many of which are based on conventional designs and hence are experimental only in name. The term "research aircraft", by contrast, generally denotes aircraft modified to perform scientific studies, such as weather research or geophysical surveying, similar to a research vessel.