The limited capacity of primates to conceptualize others as distinct and relevant individuals (from an article on Cracked.com). Studies suggest that for humans this limit is about 150 people, and those outside the monkeysphere are not really considered people at all, but one-dimensional bit characters.
Used as a term of endearment ("You are totally in my monkeysphere, baby") or indifference ("Did you hear about the mudslides in Argentina?" "Dude, that's way out of my monkeysphere. Who cares...").