Unsolved Problems in Biology/What is Life?

“Life has many different meanings, as outlined in any dictionary. It is not possible to find one universal definition covering all meanings. The best one can hope for is to find a common denominator for all these different meanings.“

Biology is centered around the phenomenon of life. As it is mostly concerned with the scientific study of life forms, biology does not lend itself to a definition of what life actually is. There are at least as many definitions of life as there are biology text books (of authors that do not simply reproduce what they have read somewhere else before). There is consensus, however, that there is no single property that delineates living from non-living systems. Life seems to rather express itself in a variety of attributes that are common to all living beings.

These commonalities include:

The exhibition of some kind of order (via a process of self-organization). For some authors this is the most remarkable aspect of life, as the general tendency within the universe is the reduction of order (i.e., an increase degrees of freedom or, physically speaking, entropy). Hence, they characterize life as negentropy - a phenomenon that temporarily and locally counteracts the universal trend of increasing entropy.
Movement/Growth/Development. That is, existence of a certain nonstationarity of biological systems over time. This includes the finite nature of an individual's existence.
Reaction/Adaptation. This evolution is another most central aspect of life. Darwinian evolution is, however, not restricted to living beings (e.g., cultural evolution).
Energy conversion/Metabolism. While this is certainly an important aspect of life (and not directly delineable from point 1), there are some biological organisms (e.g.. virii, viroids, prions that do not show any metabolism. For some authors this is reason enough to declare that a virus is a non-living entity (whose scientific study nonetheless falls into the domain of biology).
Reproduction. All known life forms (with the disputable exception of prions maybe) are based on a genetic code, with DNA as its material basis. Virus particles also consist of DNA, but are incapable of sustaining a metabolism (and hence reproduction) of their own.

Life is characterized by diversity - a product of its evolution. Thus, "life" is an abstraction of all living phenomena that share a common origin.