Life, classically, is that which acts against what is non-self to maintain itself. A reproducing locus of being that guards against non-being. It exists spatially and temporally, becoming amongst being. Life is the being that becomes; the becoming that is.
To give a scientifically precise definition to life, we can perhaps turn to Schrodinger, who famously defined life as a mechanism to reverse entropy. Entropy, to be brief, is a measure of disorder through the number of states (out of all possible states) that a thing can inhabit through the properties of a thing can roughly stay the same. Life cannot have no entropy, for that would mean that it cannot change. But it has to guard against entropy—for us, through our body mechanisms—so that it does not become a locus of undifferentiated flux, a Heraclitean non-being.
In short, life is that which lives liminally between chaos and order, between randomness and control.
But this “Life”, really, is bare and barren. It is not conscious. It has no desires. It does not think. It does not, or need not, feel. It just, so to speak, lives. Like a zombie. (And is this not why the genre of the “undead” is so popular?) This definition also helps us understand why there is such a proclivity to organicize society, to treat a conglomeration of human beings as a single life—for it in some sense is. But this perhaps shows that our definition of life is not enough, or that one has to think greater in this area of enormous complexity, to do what thought does best—create concepts and differentiate ones that have blurred into each other. I, however, have to go and finish my Biology homework. So this shall be the end…