Acronym for "Identified Flying Object".
This is distinct from
UFO, or "Unidentified Flying Object", in that one has identified the object. And, as
Robert Anton Wilson has pointed out, people see plenty of both kinds of thing every day.
For instance: I was riding my
motorcycle along a deserted
interstate in
Wyoming, when something came fluttering down from above me and to my left. I didn't know what the heck it was, and at 90 miles per hour I wasn't going to do much about it. And, yes, it turned out to be a
suicidal bird dive-bombing my helmet. With a
meaty "
thwack", that day's UFO turned into a (dead) IFO.