functional entity formed by a group of interacting, interrelated or interdependent parts
Etymology: from the Latin systēma, which in turn is derived from the Greek σύστημα meaning “together”.
[Source: adapted from Wikipedia]
Remark: We deal with holistic systems. The general principle of holism was concisely summarized by Aristotle in the paper Metaphysics: "The whole is more than the sum of its parts."