A supernatural creature whose existence can not be demonstrated, but for which certain facts are nevertheless asserted (if it's invisible, how do you know it's pink?).
Apparently orginating on the alt.athiest usenet group as a way to either argue the existence of God with non-athiests without getting personal, or mock them relentlessly. Sometimes shortened to IPU.
We know the Invisible Pink Unicorn exists, because no matter how hard hard we look we can't see her.