Familiar initialism coined by a U.S. Air Force captain

UFO

This three-letter term became widely known after a 1952 sighting report by Captain Edward Ruppelt, who led a military investigation into unexplained aerial phenomena.

This clue points to "UFO," which stands for "Unidentified Flying Object," a term popularized in the 1950s by U.S. Air Force Captain Edward Ruppelt while investigating mysterious aerial sightings. The word "initialism" hints that the answer is an abbreviation spoken as letters (U-F-O), not a word itself. The reference to the Air Force ties directly to the term’s military origins, making it a straightforward but clever nod to its history.