A relationship between two or more quantities which can be expressed in terms of linear algebraic, differential, or integral equations. A system in which all quantities (or variables) can be described in terms of such equations is said to be a linear system. By definition, linear systems satisfy the principle of superposition. By this principle, the response of a linear system to multiple inputs is given simply by the sum of the responses due to each individual input. In addition, if all inputs are multiplied by a common constant factor, the resulting response is multiplied by the same factor.