Extraterrestrial life (from the Latin words: extra ("beyond", or "not of") and‎ terrestris ("of or belonging to Earth")) is defined as life that does not originate from Earth. Hypothetical forms of extraterrestrial life range from simple bacteria-like organisms to sapient beings far more advanced than humans. It is currently unknown whether any such forms of life exist or ever existed.
The development and testing of theories about extraterrestrial life is known as exobiology or astrobiology; the term astrobiology however also covers the study of life on Earth, viewed in its astronomical context.

http://en.wikipedia.org/wiki/Extraterrestrial_life
from Wikipedia