1. any collective entity consisting of a set of interrelated or interacting elements that have been organized together to perform a function.

2. an orderly method of classification or procedure (e.g., the Library of Congress Classification system).

3. a structured set of facts, concepts, and hypotheses that provide a framework of thought or belief, as in a philosophical system. See conceptual system.

4. a living organism or one of its major bodily structures (e.g., the respiratory system). —systematic adj.