Life is a phenomenon within the context of an environment. A unique characteristic of this phenomenon is that it simply strives to continue (or you may say, survive and sustain). It derives sustenance from environment and the same environment threatens its continuity in different ways. In turn, by virtue of its characteristic of continuity, the phenomenon adapts its "implementation" (the living beings) or manifestation to give it newer forms and survival tactics. Another distinguishing characteristic of this phenomenon (to distinguish from other non-life continuing phenomena) is that it moves from the state of disorder to order over time in its local environment.
An important pre-condition that I may add here is that the environment must be natural, i.e., an environment with pre-set rules (like laws of physics which do not change for life to sustain) and randomness, not a controlled environment (like a man-made environment where humans can interfere and change conditions). It's not natural in the sense that humans can't create it but it must meet above criteria.