Formal USAF adoption of the label "unidentified flying objects" definitely occurred no later than early 1952, when Project Blue Book was established. Project Blue Book's first head - Capt. Edward Ruppelt - is widely credited as the person who developed and instituted the notion of "unidentified flying object" as formal terminology.
I have coincidentally, only yesterday, come across an exceptional and related snapshot, published in a feature article by 'Look' magazine, on 1 July 1952.
It covers three pages and I have transcribed the page one introduction:
Whatever it is, the Air Force must HUNT FOR THE FLYING SAUCER
This map scared the Pentagon. It pinpoints unexplained "flying saucer" sightings across the nation with concentrations near vital defense installations.
Fearful of danger from the skies, the United States Air Force is launching a secret scientific search to discover once-and-for-all what is the mysterious, unbelievable thing Americans keep sighting overhead.
By J. ROBERT MOSKIN
The U. S. Air Force, entrusted with the defense of the United States from the skies, is mounting a new hunt for the mysterious, seen-but-never-caught "flying saucers" which for five years have had Amerieans excited, baffled and even terrified.
During the past two weeks, Air Force officials have begun experiments to verify the radically new theory of Dr. Donald H. Menzel of Harvard University. This nationally-known astronomer suggested in the last issue of Look that flying saucers are actually optical illusions caused by "temperature inversion" which projects images of earthly lights on the sky. This has already proved one of the most exciting explanations of flying saucers offered to the Air Force.
Asked about the Air Force's revived concern over flying saucers, after almost ignoring them for two years, Gen. Hoyt S. Vandenberg, chief of staff, in his first public statement on the subject, told Look:
"The Air Force is interested in anything that takes place in the air. This includes the aerial phenomena commonly known as 'flying saucers.' Many of these incidents have been satisfactorily explained. Others have not. With the present world unrest, we cannot afford to be complacent."
The Air Force has collected more than 800 sightings of flying saucers, and reports continue to flow in from our outposts like Alaska and Newfoundland and from our vital atomic installation sites. They are spurring the Air Force to seek answers to the saucer puzzle in a hurry.
This is a job for the detectives of the Air Force, the officers of the Air Technical Intelligence Command, working behind a shield of secret planning and classified documents at heavily-guarded Wright-Patterson Air Force Base outside of Dayton, Ohio. Now for the first time exactly what Air Force Intelligence is doing about flying saucers can be revealed.
As one highly placed intelligence officer put it, "Our job is to detect any weapon that might be used against the United States. In the future, a weapon that 'probably was a meteor'. may prove to be a global rocket."
Visual sighting reports can never give Intelligence accurate data on speeds, altitudes, size and composition of the objects seen. To get these facts, a methodical and scientific plan of great breadth has now been blueprinted. It includes these steps:
1. Under ATIC direction, a physicist at the University of California at Los Angeles is developing and testing a special camera to photograph flying saucers. Key to the new apparatus is a defraction grid consisting of a piece of glass etched with infinitely fine lines. Placed over the camera lens, this grid breaks down the image into slivers from which scientists can determine its composition. If the saucers prove to be bodies which glow, the grid will record the material they are made of. If their light comes from a fuel supply or a reflection, the grid will identify the light.
(...)
The full and fascinating article can be seen here:
www.forteanmedia.com/Look.pdf