The reaction or result produced by applying an input signal to a system. Given a system, or interconnection of devices to perform a desired function, the signals processed by that system can be divided into several categories, some of which are referred to as causes, stimuli, or input signals; and others, as responses, effects, or output signals. Although there can be multiple input and output signals for a given system, attention will be focused on single-input single-output systems. Often, the input and output signals of interest are functions of time only, although the discussions given can be generalized to more than one independent variable, such as space and time. Also, attention will be limited to continuous-time signals and systems, although the discussion can be easily put in terms of discrete-time signals and systems.