In the context of aviation and aerospace, an "aircraft" refers to any machine that is capable of sustained flight through the atmosphere, using aerodynamic forces to generate lift and thrust. This can include a wide range of vehicles, from small unmanned drones to large commercial airliners, and from military fighter jets to spacecraft that can operate in the atmosphere.

Aircraft can be broadly classified into several categories, including:

1. Fixed-wing aircraft: These have wings that are fixed in place and use lift to generate upward motion. Examples include airplanes, gliders, and sailplanes.
2. Rotorcraft: These have rotating wings or rotors that generate lift and thrust. Examples include helicopters, gyrocopters, and tiltrotors.
3. Lighter-than-air craft: These use buoyancy to generate lift and are typically filled with gases such as helium or hydrogen. Examples include balloons, airships, and blimps.
4. Unmanned aerial vehicles (UAVs): These are remotely controlled or autonomous aircraft that are used for a variety of purposes, including reconnaissance, surveillance, and cargo transport.
5. Spacecraft: These are vehicles that are designed to operate in the vacuum of space, but may also be capable of atmospheric flight. Examples include the Space Shuttle and reusable rockets.

Aircraft can be powered by a variety of means, including:

1. Reciprocating engines: These use a piston and cylinder arrangement to generate power.
2. Turbine engines: These use a turbine to generate power and are commonly used in commercial airliners.
3. Electric motors: These use electrical energy to generate power and are increasingly being used in UAVs and other small aircraft.
4. Rocket engines: These use combustion or other reactions to generate thrust and are commonly used in spacecraft and military missiles.

Overall, the term "aircraft" encompasses a wide range of vehicles that are designed to operate in the atmosphere, and the specific characteristics and capabilities of an aircraft can vary widely depending on its intended use and design.