An antimatter torpedo with adjustable yield and far greater range than spatial torpedoes, installed aboard Enterprise NX-01 before its mission into the Delphic Expanse.
Photonic torpedoes were the first Starfleet weapons to carry matter and antimatter warheads. The Klingons were already using similar torpedoes when Enterprise examined a stricken Raptor in the episode Sleeping Dogs.
In The Expanse, after the Xindi attack on Earth, Enterprise returns home for a refit and receives the new weapons. Malcolm Reed explains that their range is many times that of the old spatial torpedoes and that the yield can be dialled from a blow that barely dents a hull to one that can crater an asteroid. They are first fired in anger at Duras's Bird-of-Prey.
The torpedoes serve throughout the Xindi mission and afterwards, and are the direct ancestors of the photon torpedoes of later centuries.