A starship is any manned spacecraft that is capable of viable interstellar travel, i.e any manned spacecraft that has the ability to transport and support a crew across interstellar distances in a reasonable time. A starship accomplishes this via the employment of some form of faster-than-light spacecraft propulsion technology, such as warp drive.
The term starship is often interchangeable with spacecraft or spaceship. Though the latter two terms can describe crafts capable of any sort of space faring flight. This can range from crafts designed for simple orbital to more advanced space travel, in designs ranging from probes, to shuttles and spaceships.