Life is an illusion. Now I'm really starting to feel that six-pack. Let's set aside the question of sentient life to avoid arguments about the soul. It seems obvious that at some level all we see about us, living or otherwise, is merely a manifestation of chemical reactions and the laws of physics. In short, life is an arbitrary distinction.
Life reverses local entropy.
Popularized by Isaac Asimov. In lay terms, life reverses the default trend toward ever-greater disorganization. Still, this one's got a certain appeal. In contrast to, say, fire, which in its uncontrolled form is one of your more basic entropic phenomena, life is a creative force. Admittedly that idea isn't much help in deciding what's alive and what ain't.