Maybe an odd question for an astrophysics FAQ, but of course it’s a question that we need to answer if we’re going to look for aliens!

It has been surprisingly difficult to get scientists to agree on a universal definition for life; however, in very general terms, most would accept that life must at least 1) reproduce (make copies of itself) and 2) engage in some form of metabolism (cause the conversion of chemicals from one form to another for use by the organism). Wikipedia has a very reasonable discussion on the arguments over the definition of life and offers a more rigorous definition.