In Star Trek, what weapon launches self-guided explosive projectiles from a starship?

In Star Trek, photon torpedoes are self-guided explosive projectiles launched from starships.

A photon torpedo is a spacecraft weapon that combines a matter–antimatter warhead with a propulsion and guidance system. The antimatter reaction releases enormous energy when matter and antimatter annihilate, producing the destructive effect associated with the weapon. Torpedoes can be fired at ships, stations, or planetary targets.

Photon torpedoes are not the same as phasers. Phasers are directed-energy weapons that fire beams or pulses, while torpedoes are physical projectiles that travel through space. Starfleet vessels commonly carry both systems because each has different tactical uses.

The weapon was introduced in the original Star Trek era and remained standard equipment across later series and films. Later stories also feature quantum torpedoes, which are a more advanced weapon associated particularly with the 24th-century Starfleet. That later type should not be used as the answer when a question specifically asks for the earlier standard weapon.