A microtorpedo was a type of weapon developed by Starfleet in the 24th century. They were designed for use on Danube-class runabouts to supplement the vessels' limited armament of six phaser strips. (DS9 novel: Antimatter)
In January 2377, Taran'atar placed the microtorpedoes from the USS Euphrates on the comet Nahanas, along with Prynn Tenmei's combadge. When the crew of the USS Defiant lowered their shields and attempted to beam Prynn aboard, the microtorpedoes detonated, destroying the comet. (DS9 novel: Warpath)
The Delta Flyer also equiped with these torpedos.