In general terms, the word alien is a reference to something originating from outside the environment in which it is found. Such environments can be of a cultural, geographical or physical nature. For example, the term illegal alien commonly refers to a human being from one nation who has taken up residence in a different nation without legal permission. In botany and zoology an alien species is one that is non-native to the region in which it is found.

In the context of ufology, because the prevailing view is that UFOs are a global phenomenon that cannot reasonably be attributed to any nation on Earth, they are therefore from beyond boundaries and constructs of our civilization. This situation makes them alien to our collective global civilization, but not necessarily extraterrestrial or non-human in origin. More exotic alien possibilities include extradimensional, extratemporal, submarine or subterranean. Alien beings from space or another planet are called extraterrestrials ( ETs ), also sometimes referred to as extraterrestrial biological entities ( EBEs ). However aliens could also be android or robotic. It has often been speculated that some MIB encounters have involved androids. Alien subject matter can include living beings, artifacts, communication, knowledge or technology.

Most astronomers and astrobiologists agree that intelligent life probably exists elsewhere in the universe, but that it is very unlikely that any have visited Earth. One of the most noted astronomers who became involved in ufology and influenced its course was Carl Sagan. Sagan began with a sympathetic view toward the idea of alien visitation but eventually conceded to the pressure of the scientific establishment and became a noted skeptic. In contrast, the astronomer J. Allen Hynek , started out as a skeptic and after spending many years studying the UFO phenomenon became convinced that UFOs are real objects. Both astronomers believed that UFOs deserve serious scientific study.