- UFO (Unidentified Flying Object): Any flying object or phenomenon that cannot be identified by the observer.
- Extraterrestrial Life: Life that originates outside of Earth.
- Close Encounter: A term used to categorize encounters with UFOs or their occupants. A close encounter of the first kind is a sighting, the second kind is physical evidence, and the third kind is contact.
- Alien Abduction: The phenomenon of people reporting being abducted by extraterrestrial beings.
- Crop Circles: Patterns created by the flattening of a crop. Some people believe these are created by extraterrestrials.
- Roswell: A city in New Mexico, known for the Roswell UFO incident in 1947, when a rancher discovered unidentifiable debris, sparking widespread speculation about extraterrestrial life.
- Area 51: A highly classified remote detachment of Edwards Air Force Base within the Nevada Test and Training Range in the United States. Conspiracy theories frequently suggest it’s a place where the government stores secret information about extraterrestrial life.
- SETI (Search for Extraterrestrial Intelligence): A scientific effort to detect intelligent life elsewhere in the universe, primarily by listening for radio signals.
- Fermi Paradox: The apparent contradiction between the lack of evidence for extraterrestrial civilizations and the high probability estimates for their existence.
- Drake Equation: A probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.
- Grey Aliens: An alleged extraterrestrial being, characterized by their grey skin and large eyes. They are often associated with alien abduction stories.
- Little Green Men: A stereotypical depiction of extraterrestrials as little humanoid-like creatures with green skin.
- Disclosure: A term used by UFO researchers to refer to the long-awaited event when the government officially admits to the existence of extraterrestrial life.
- Men in Black: Mysterious individuals, often associated with UFO sightings, thought to be government agents sent to keep witnesses quiet.
- Ancient Astronauts Theory: The idea that extraterrestrial beings visited Earth in the ancient past and significantly influenced the development of human cultures, technologies, and religions.
- Project Blue Book: A series of systematic studies of UFOs conducted by the United States Air Force from 1952 to 1969.
- Flying Saucer: A term often used to describe UFOs, particularly those that are disc-shaped.
- UAP (Unidentified Aerial Phenomena): A term used as a broader, more scientific term for UFO, to remove the stigma associated with the ‘UFO’ label.
- Cryptozoology: The study of creatures, such as Bigfoot or the Loch Ness Monster, whose existence is not proven.
- Interdimensional Hypothesis: The idea that UFOs and related events involve visitations from other “realities” or “dimensions” that coexist separately alongside our own.