Acronym for Unidentified Flying Object: a term used to refer to sightings of airborne craft whose identity is uncertain. Some instances may be of alien origin; then again, maybe not. In fact a UFO cannot be an alien spaceship by definition, because once identified as such it ceases to be an Unidentified Flying Object.
More often than not, an investigation of the evidence will reveal the objects to be car hubcaps, thrown into the air and photographed - badly.
They also tend to gather around Area 51 on the weekends.