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.
In the Federation, during the 23rd century, starships were identified exclusively as vessels operated by Star Fleet. Within the Federation, starships were viewed as the role of protector within the civilian community, as starship captains were given a certain level of authority in the region they patrolled. While generally limited to the roles of exploration and defense, starships typically do not transport cargo, 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, as a vessel utilizing 100% power has the ability to skew sensor readings.
By the mid-24th century, the exclusiveness of term starship would essentially fall out of use, and would be redefined as a term interchangeable with spaceship and spacecraft.