UFO

UFO was most commonly used on Pre Astro Earth as an initialization of the term "Unidentified (or Unknown) Flying Object." While that term could be used to describe any object "flying in the air", prior to the Sarans landing on the White House Lawn in 2089, the term often referred to speculated to refer to ships piloted by visitors from other planets.

It's most common current use is as an initialization of "Unidentified Fried Object" referring to fried food that is not readily identifiable.