What does halo stand for in the video game

Halo doesn't stand for anything. it means the halo rings that the forerunners used to destroy the flood. then later Master Chief used it to destroy the flood also.

OR

H.A.L.O. stands for
High
Altitude
Low
Opening

Which makes reference to the altitude that the special ops team jump out of the deployment aircraft and the low altitude (after which) that they deploy their parachutes.