(computing) An old computer system or application program that continues to be used because the user (typically an organization) does not want to replace or redesign it.

(logic) A formal system.

(logic) The grouping of a formal language and a set of inference rules and/or axioms.

(philosophy) A consistent set of moral axioms or principles.

A computer system that uses techniques derived from artificial intelligence, particularly one in which such techniques are central to the operation or design of the system.

(computing) A new computer part that has been removed from an unused, previously assembled system.

(Britain, Canada) Alternative spelling of honor system

(law) A system of law in which advocates represent the prosecution and the defence and an impartial person (judge) or people (jury) determine the truth of the matter

(physics) A localized physical system consisting of interacting components, called degrees of freedom

(astronomy) A heliocentric model of the solar system, proposed in the 16th century by the eponymous astronomer.