''Unidentified- flying- object." 

An internal report from Project Blue Book, the USAF program that dealt with the UFO ''problem''.
Special Report No. 14 was an statistical analysis of U.F.O. report cases gathered from 1947 to 1954, some 3,200. It represents the largest such study ever undertaken.  Cases were divided into three categories-"knowns", "unknowns", and "insufficient information."
Blue Book was cancelled  in 1968. 

Project Blue Book Special Report #14 (USAF Air Technical Intelligence Center, 1954)

https://archive.org/details/ProjectBlueBookSpecialReport14