From Halopedia, the Halo wiki
The Killimanjaro or Killamanjaro medal is awarded for killing seven opponents within four seconds of each other in standard multiplayer games (and Halo: Spartan Assault) and within less time in campaign and Firefight. In Halo 4 multiplayer and Spartan Ops, the kills need only be within 4.5 seconds of each other.
In Halo 2 through Halo: Reach the Killimanjaro medal is represented by seven yellow stars on a platinum circle. In Halo 4 the Killamanjaro medal is represented by seven white stars on a yellow-green circle rimmed with yellow and brown. The Killamanjaro medal is monochrome and imposed on a shield in Halo: Spartan Assault and Halo: Spartan Strike. When someone earns this medal, the announcer will shout “Killimanjaro!” to the player that received this medal.