Avatar
From Halopedia, the Halo wiki
An Avatar is the physical representation of an Artificial Intelligence or AI, used as a way to better interact with Humans.
While not used by all AIs, every "smart" AI uses an avatar (though many "dumb" AIs do). This physical representation is chosen by the AI upon their creation and can be whatever they desire it to be, although the duty which the A.I. is to perform, combined with the Human brain that was used to create it, (like Melissa) have a great deal to do with what avatar they chose to appear as.
For more detailed information of known AIs that use avatars, please see the list of the Artificial Intelligence serving the United Nations Space Command.