Plasma Kill is a medal first awarded in Halo 4 multiplayer for killing an enemy with a Plasma grenade. Unlike the Grenade Stick medal found in previous games, the Plasma Kill medal does not require the player to stick their enemy with the plasma grenade; killing an enemy with a plasma grenade's splash damage will suffice.
The medal returns in Halo: The Master Chief Collection where it is awarded in Halo: Combat Evolved, Halo 2, Halo 2: Anniversary and Halo 4 multiplayer, but not in Halo 3 multiplayer or in any of the campaigns. The Grenade Stick medal also appears in the collection, and is awarded in all multiplayer modes save for Halo 4. Thus in all multiplayer modes save for Halo 4, the medal is only awarded for kills with plasma grenades that are not sticks.