A plasma bullet
Plasma bullets were the concentrated plasma discharges produced by some types of plasma weapons.
In 2151, Subcommander 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")
Based on the special effects in ENT: "Broken Bow", the ammunition fired by the EM-33 pistols also appeared to be plasma bullets.