ABSTRACT
Defining life is important for astrobiology for several fundamental reasons. Many specific aspects of astrobiology also require a definition of life. In this chapter, the authors give a short background on definitions in general and provide selected examples of the definitions of life. Different types of definitions exist, such as lexical, stipulative, and operational definitions. For any definition of life to be astrobiologically significant, it must include an explanation of the origin of life. Life as a system has some characteristics in common with systems that are not alive, such as an engine. Life’s subsystems are composed of subsidiary subsystems. Modern rovers have such manifold capabilities that they manifest almost as a living organism. Communications for the human is multi-faceted and widely distributed. Different bacterial species and strains have different optimum temperatures for growth and reproduction. The ability to translocate the center of mass of an organism is distinct from the rooted nature of the higher plants.
