A continuous-time signal is said to be a **right-sided signal** if there exists a T such that for all t < T, x(t) = 0. A discrete-time signal is said to be a **right-sided signal** if there exists a T such that for all n < T, x[n] = 0.

A continuous-time signal is said to be a **left-sided signal** if there exists a T such that for all t > T, x(t) = 0. A discrete-time signal is said to be a **left-sided signal** if there exists a T such that for all n > T, x[n] = 0.

A continuous-time signal or discrete-time signal is said to be a **two-sided signal** if it is not a left-sided signal and it is not a right-sided signal.