The perfection in virtue of which an agent is capable of immanent action.
Bodies as natural units are found to be possessed of various kinds of activity.
Organic or living bodies have an organized structure of heterogeneous parts; inorganic bodies are homogeneous in structure.
Organic bodies through the action of the physical and chemical forces inherent in them produce effects which always pass from the agent to some object distinct from it; these activities are called transient.
The organized bodies, however, besides exercising transient activities are endowed with other activities never found in the inorganic, e.g., nutrition, growth, and reproduction; some organized bodies are also capable of consciousness and the various local motions arising therefrom; living man is conscious of forming judgments, of reasoning, and of striving for the attainment of non-sensible good.
All of these activities are living, vital, and in the corporeal universe are found to be the exclusive properties of bodies which we call living.
The definition of life, then, must be found in some quality common to all of these functions and to these alone.
Analysis shows that everyone of them results in a term which remains, and must of its nature remain, as a perfection of the natural unitary whole producing it.
Hence the name immanent activity, also called self-movement.
It involves three essential elements: the unit of activity must be a natural unitary whole, not merely an artificial unit; the efficient cause immediately eliciting the activity must be a power within this unit; the immediate term of the activity must remain as a perfection of the unit.
Thus an organism in nutrition by its own active power produces as a term the anabolism within the cells comprising the organism.
New Catholic Dictionary