UAP stands for unidentified aerial phenomenon (pl., phenomena), a 21st Century substitution intended to overcome the ridicule assigned to UFOs, or Unidentified Flying Objects, of the 20th Century.
"The unidentified aerial phenomenon" can refer to the existence of craft in the skies in general. The abbreviation is more likely to be used for the appearance of a single craft, with "UAPs" used to refer to multiple sightings.
- E.g., The Phenomenon (2020), Nuclear Weapons and the Unidentified Aerial Phenomenon: A Call for Open Hearings on the National Security Implications, annotated here
- Greg Eghigian. airspaceemag.com, August 5, 2021: UFOs, UAPs—Whatever We Call Them, Why Do We Assume Mysterious Flying Objects Are Extraterrestrial?