Here’s a little outline of the scientific issues involved in determining what life is.
The succinct definition in that article:
Life (cf. biota) is a characteristic that distinguishes objects that have signaling and self-sustaining processes (i.e., living organisms) from those that do not, either because such functions have ceased (death), or else because they lack such functions and are classified as inanimate.
One of the key questions is whether viruses are alive or not. Wikipedia says:
Viruses are most often considered replicators rather than forms of life. They have been described as “organisms at the edge of life,” since they possess genes, evolve by natural selection, and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not metabolize and require a host cell to make new products. Virus self-assembly within host cells has implications for the study of the origin of life, as it may support the hypothesis that life could have started as self-assembling organic molecules.