The greeting given to co-workers who pass by in a hallway at work.
Hallutations invariably progress from a friendly "good morning" or "hey" at the first encounter of the day with a given co-worker, to a quick nod of the head, then to the subtle 'eyebrow nod', and finally to an aggressive refusal to acknowledge the co-worker's existence for any more passings.
At the end of the day, this progression resets, and the next morning hallutations are again initiated in the friendly "hey" stage.
Eventually, Bob dreaded the walk to the bathroom, as it required him to recall the exact stage of hallutation for each of the co-workers he might encounter in the long hallway.