Image from Kentsuku CG World (copyright; used with permission)
Generally, a heavily armed and armoured, short-range military vessel, capable of operating in a hydrospheric, atmospheric, and exoatmospheric (vacuum-capable) environment for prolonged periods of time.
Gunships are used primarily to provide fire-support for ground troops, but can also take on enemy fighters and ships, and may also be used for bombing enemy installations. Although not capable of long-range interplanetary operations, and no match for larger vessels or ultratechkhaki goo, they can continue local resistance for long periods, hiding in oceans or local gas giants, and coming out only when there is promise of enemy losses.
Subcraft or subships are Gunships specially designed for extended lurking sudden ambush tactics. A Lurkship is a larger and better equipped version.