There is no complete explanation of life; there are a diversity of definitions proposed by different scientists. To define life in indisputable terms is still a challenge for scientists.

Usual definition: Often scientists say that life is a characteristic of organisms that show the following phenomena:

1. Homeostasis: instruction of the internal environment to maintain a regular state; for example, sweating to reduce high temperature.

2. Organization: Being composed of one or more cells, which are the vital units of life.

3. Metabolism: Consumption of energy by converting nonliving material 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 synthesis than catalysis. A growing organism increases in mass in all of its parts, quite than purely accumulating matter. The particular species begins to multiply and get bigger as the progression continues to flourish.

5. Adaptation: The capability to modify over a period of time in reply to the environment. This ability is essential to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and outside factors present.

6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning near the sun or an animal chasing its prey.

7. Reproduction: The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although severely speaking it also describes the invention of new cells in the process of growth.

However, others cite some limitations of this definition. Thus, many members of several species do not reproduce, possibly because they belong to specialized sterile castes (such as ant workers), these are still considered forms of life. One could say that the possessions of life is hereditary; hence, sterile or hybrid organisms such as the mule, liger or eunuchs are alive although they are not capable of self reproduction.

Proposed definitions of life include:

1. Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation.

2. Life is a characteristic of self-organizing, self-recycling systems consisting of populations of replicators that are capable of mutation, around most of which homeostatic, metabolizing organisms change.

The above definition includes worker caste ants, viruses and mules while precluding flames. It also explains why bees can be alive and yet commit suicide in defending their hive. They are only individual instances of the living structure that comprises all life forms on planet Earth (which is the only living system known to mankind).

1. Type of organization of matter producing various interacting forms of variable complexity, whose main property is to replicate almost completely by using substance and power accessible in their environment to which they may adjust. In this definition "almost perfectly" relates to mutations happening during imitation of organisms that may have adaptive benefits.

2. Life is a potentially self-perpetuating open system of linked natural reactions, catalyzed at the same time and almost isothermally by multipart chemicals (enzymes) that are themselves produced by the open system.

Of course we need to admit that our concept of life is based on our own perception of the universe. We can practice that we are living and from there we enlarge the theory of life with forms, entities with similar properties, like flora and fauna. As it was discovered how we are made up out off cells, being made up out off cells has by some been qualified as a indispensable property of life. But, as illustrated above, this is probably not the case when speaking of more hypothetical and non-traditional forms of life, thus also other properties could be an indication for life, like for example a certain form of sentience, conscience, intelligence and/or sapience. Thus the definition of life is rather made up out of multiple possibilities of life to exist, by some qualities which are unified in human life (although it needs to be considered that some possibilities might not be represented in humans, in this case it could be problematic to conclude whether it is really living or not).
But all these possibilities might theoretically also lead to a form of life on their own.

Source : www.wikipedia.org