Final answer:
A small unmanned aircraft is a lightweight and compact aircraft that does not carry a pilot on board. The LightSail spacecraft consisting of CubeSats is an example of a small unmanned aircraft.
Step-by-step explanation:
A small unmanned aircraft can be defined as a lightweight and compact aircraft that does not carry a pilot on board. It is typically controlled remotely or preprogrammed to fly autonomously. In the context of the given information about the LightSail spacecraft, the spacecraft consisting of three CubeSats bundled together can be considered as a small unmanned aircraft. It has a total mass of about 5 kg and is roughly the size of a loaf of bread.