Life is a characteristic of organisms that exhibit certain biological processes such as chemical reactions or other events that results in a transformation. Living organisms are capable of growth and reproduction, some can communicate and many can adapt to their environment through changes originating internally.  A physical characteristic of life is that it feeds on negative entropy.  In more detail, according to physicists such as John Bernal, Erwin Schrödinger, Eugene Wigner, and John Avery, life is a member of the class of phenomena which 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 (see: entropy and life).

An entity with the above properties is considered to be a living organism, hence, a 'life form'. However, not every definition of life considers all of these properties to be essential. For example, the capacity for evolution is sometimes[citation needed] taken as the only essential property of life; this definition notably includes viruses, which do not qualify under narrower definitions as they are acellular and do not metabolize.

A diverse array of living organisms can be found in the biosphere on Earth. Properties common to these organisms—plants, animals, fungi, protists, archaea and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information. They undergo metabolism, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. So far, there is no evidence of extraterrestrial life.