From wikipedia:
Biology
Since there is no unequivocal definition of life, the current understanding is descriptive. Life is considered a characteristic of organisms that exhibit all or most of the following characteristics or traits:[32][34][35]
1.Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
2.Organization: Being structurally composed of one or more cells â" the basic units of life.
3.Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
4.Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
5.Adaptation: The ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.
6.Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
7.Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.
These complex processes, called physiological functions, have underlying physical and chemical bases, as well as signaling and control mechanisms that are essential to maintaining life.
Alternatives
To reflect the minimum phenomena required, other biological definitions of life have been proposed,[36] many of these are based upon chemical systems. Biophysicists have commented that living things function on negative entropy.[37][38] In other words, living processes can be viewed as a delay of the spontaneous diffusion or dispersion of the internal energy of biological molecules towards more potential microstates.[39] In more detail, according to physicists such as John Bernal, Erwin SchrÃdinger, Eugene Wigner, and John Avery, life is a member of the class of phenomena that are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form.[40][41][42] At a higher level, living beings are thermodynamic systems that have an organized molecular structure.[39] That is, life is matter that can reproduce itself and evolve as survival dictates.[43][44] Hence, life is a self-sustained chemical system capable of undergoing Darwinian evolution.[45]
Others take a systemic viewpoint that does not necessarily depend on molecular chemistry. One systemic definition of life is that living things are self-organizing and autopoietic (self-producing). Variations of this definition include Stuart Kauffman's definition as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle.[46] Life can be modeled as a network of inferior negative feedbacks of regulatory mechanisms subordinated to a superior positive feedback formed by the potential of expansion and reproduction.[47] Alternatively, life can be said to consist of things with the capacity for metabolism and motion,[32] or that life is self-reproduction "with variations"[48][49] or "with an error rate below the sustainability threshold."[49]
Viruses
Electron micrograph of adenovirus with a cartoon to demonstrate its icosahedral structureViruses are most often considered replicators rather than forms of life. They have been described as "organisms at the edge of life,"[50] since they possess genes, evolve by natural selection,[51][52] and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not metabolize and they 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.[53][54][55]