system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] Why is the main land force of the Imperium of Man called the "Imperial Guard" and not the "Imperial Army"?
gpt
They were originally known by that name. Transition came mostly in the wake of the Horus Heresy as the Imperial Army was broken up and restributed to different branches, creating the Imperial Navy, Imperial Guard, and the other minor branches intended to limit overall authority and compartmentalize forces to prevent something like the Heresy from happening again.

There is some indication that the title of Imperial Army was still in use after this restructuring, but was replaced with the honorific Imperial Guard, after an unknown battle in which service members distinguished themselves as guardians of the Imperium. 

Whatever the case though, the title is ultimately an honorific. Just like the Astartes are the Angels of Death, it's not an official name, but an earned title. They are the Imperial Guard because they are the stalwart guardians of the Imperium of Man.