From paleo, meaning old or ancient and SETI, the Search for Extraterestrial Intelligence
Dr. Vladimir Avinski coined the term paleo-SETI in 1989 to describe the area of research dealing with the possibility that alien visitors have been coming to Earth for millenia. Erich von Däniken first popularized the idea of these "ancient astronauts" in his 1969 book Chariots of the Gods, and in later books embraces Avinski's term as his "paleo-SETI philosophy."
In nearly every culture, there are myths involving gods or other powerful, mystical beings descending from the heavens to teach mankind. These beings return to the heavens when their task is complete, but promise to return again one day. Many ancient structures are attributed to these entities, either built by them or at their instruction, as reminders or storehouses of information for future generations. When we have achieved a sufficient level of technical advancement, we will be able to decipher this information and put this legacy to use.
The basic tenet of paleo-SETI research is that there is at least a kernel of truth in these legends, that they refer to actual historical events. These mystical beings, however, may not have been gods and divine messengers; instead they might have been alien visitors making contact with our ancestors for reasons of their own. They may even have "created" us or seeded the Earth with our genetic material. With this in mind, tales like the apocryphal Book of Enoch, the destruction of Sodom and Gomorrah, structures like the Pyramids, Stonehenge, and the Nazca Lines all take on a new, and more science fiction, feel.
Many disciplines make up paleo-SETI research. Archaeology, with specialties like Egyptology, is an obvious inclusion, expecially considering that many paleo-SETI researchers believe the Pyramids of Giza and the Sphinx are far older than conventional dating has suggested and that similarities in the legends of ancient peoples world-wide indicate either previously unknown contact between these groups or that they descend from a single, far more ancient, civilization. Evolutionary biology and genetics try to determine if aliens influenced our very rise from lower primates or even seeded the planet with primitive DNA, while astronomers, mathematicians, historians, and theologians can fill in other information.
Groups like the Archaeology, Astronautics, and SETI Research Association strive to make this line of research into an accepted area of scientific inquiry, but have met with a great deal of resistance from most scientists. Unfortunately, while for many researchers it is a serious endeavor, honestly searching for answers one way or another, others seem to take this theory as fact, and twist any information they come across to fit their theories. Mainstreaming such an off-the-wall (according to many) line of research is made doubly difficult by the way proponents like von Däniken have sensationalized it, both through popular books and places such as his Mysteries of the World theme park.
Sources:
The Archaeology, Astronautics, and SETI Research Association (AAS RA) - http://www.aas-ra.org/
http://members.surfeu.de/tomtomatic/
Chariots of the Gods and The Return of the Gods by Erich von Däniken