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 propulsion technology, such as warp drive.
The term normally refers to large, multi-purpose vessels (such as Starfleet's Class I Starships), as opposed to vessels such as cargo ships, transports, warp shuttles, and runabouts, which are designed for more specific or limited functions.
While generally limited to the roles of exploration and defense, starships typically do not transport cargo (other than that necessary to maintain the vessel and sustain the crew), except in the case of an emergency.
Design is also a factor in defining a starship. The configuration of a vessel, as identified by sensor scans, can be used in identifying a starship. Likewise, a starship can be identified on sensors by power utilization curve.