A rally point, more commonly referred to as RP, is a term used by military personnel to tell others where to meet up during a mission. Rally points may also tell soldiers where a new landing zone is, this way soldiers can be extracted from their missions safely.
Introduced in Halo 3 and reappearing in Halo: Reach, a Rally Point is a checkpoint that divides a campaign mission based on objectives. A mission can have a maximum of three Rally Points marked as "Mission Start" being the start of a mission, "Rally Point Alpha," being the halfway point of the mission in short missions with only two rally points, and one third of the mission being completed in longer missions. Opening cutscenes and training missions, such as Arrival in Halo 3 and Noble Actual in Halo: Reach, both have only a Mission Start. Halo 3: ODST does not have any Rally Points due to the short campaign missions. Lone Wolf in Halo: Reach has a single rally point, "Rally Point Omega," due to the level's unique nature. In Halo 4, "Mission Start" has been dropped in favor of starting with "Rally Point Alpha" and was replaced by "Start" for the Prologue and Epilogue.
In Halo Wars, a rally point is a flag placed on the map that is used as a point that newly created units go to when they spawn. There are normal rally points, which are plain flags in the ground and serve as points for one base or Elephant, and there are global rally points, which have two flags hanging down on either side of the pole and display the Legendary icon on the front.