A plasma torpedo is a short-ranged, high-yield weapon used by the Romulan Star Empire in the 23rd century. First seen in the TOS episode "Balance of Terror", these weapons were extremely effective against Federation starbases along the Neutral Zone.
During a briefing following the initial attacks on Neutral Zone outposts, Spock described what was known about the Romulan weapon. According to Starfleet researchers, the weapon was an "enveloping plasma" that caused its targets to implode. One shot plasma torpedo was sufficient to destroy the shields and weapons of a Federation outpost, and a second torpedo shattered the asteroid on which the outpost was built. One torpedo also damaged the fully-shielded Enterprise, even though much of the torpedo's energy had dissipated due to range.
Even though it is capable of overtaking a Constitution-class starship in full retreat, a plasma torpedo has a relatively limited range. Only cloaking technology enabled the Romulan ship to get into effective range of its targets. As noted above, a plasma torpedo lost so much energy pursuing the fleeing Enterprise that when it finally struck, it no longer had enough energy to destroy the target.
Lieutenant Sulu theorized that a single phaser shot would be sufficient to detonate a plasma torpedo prematurely. He did not actually get to test his theory, however, since his ship's phasers were off line at the time.
The Romulans stopped using plasma torpedoes after a technology exchange with the Klingons. In the 24th century, Romulan ships carry disruptors and photon torpedoes. The range limitations and high energy consumption of plasma torpedoes may have lead to their being replaced.