An unidentified flying object, or UFO, is any real or apparent flying object which cannot be identified by the observer and which remains unidentified after investigation.

Sightings of unusual aerial phenomena date back to ancient times, but reports of UFO sightings started becoming more common after the first widely publicized U.S. sighting in 1947. Many tens of thousands of such claimed observations have since been reported worldwide, and it is very likely many more go unreported due to fear of public ridicule because of the  social stigma created around the UFO topic.

In popular culture throughout the world, UFO is commonly used to refer to any hypothetical alien spacecraft but the term flying saucer is also regularly used. Once a UFO is identified as a known object (for example an aircraft or weather balloon), it ceases to be a UFO and becomes an identified object. In such cases, it is inaccurate to continue to use the acronym UFO to describe the object.