The plasma torpedo is a Romulan energy weapon that releases a destructive ball of plasma, first encountered by Starfleet in 2266.
In 2266 a Romulan bird-of-prey crossed the Neutral Zone and destroyed Earth outposts with a new weapon that unleashed a powerful bolt of plasma. The weapon was devastating at close range, but its energy dissipated over distance, and Captain Kirk escaped one attack by outrunning it until it lost strength.
That incident marked the first time anyone in the Federation saw a Romulan face, and plasma weapons remained associated with Romulan design afterwards. Other powers have also employed plasma-based torpedoes.