Plasma bullets were the concentrated plasma discharges produced by some types of plasma weapons, manifesting as a bolt of energy.
In 2151, Sub-Commander T'Pol jumped in front of a plasma bullet that had been fired at Captain Sopek by a Coridanite rebel. Captain Archer would later use T'Pol's selfless act as an excuse as to why she should be allowed to remain aboard Enterprise NX-01. (ENT: "Shadows of P'Jem")
According to the script of ENT: "Broken Bow" the ammunition fired by plasma rifles and the EM-33 pistols were also plasma bullets. [1]