Life may (somewhat irreverently) be defined in general terms as:

"The phenomenon associated with the replication of self-coding informational systems",

or specifically as:

"The phenomenon associated with the replication of nucleic acids".

- Rybicki EP, 1996.

Another more serious view:

"Life can be viewed as a complex set of processes resulting from the actuation of the instructions encoded in nucleic acids. In the nucleic acid of living cells these are actuated all the time; in contrast, in a virus they are actuated only when the viral nucleic acid, upon entering a host cell, causes the synthesis of virus-specific proteins. Viruses are thus "alive when they replicate in cells, while outside cells viral particles are metabolically inert and are no more alive than fragments of DNA."

- Dulbecco R and Ginsberg HS, 1980. Virology, p.854-855 (originally published as a section in Microbiology, 3rd Edn., Davis et al., Harper and Row, Hagerstown).