A wave that has been reflected by a discontinuity in the propagation medium. 2. A wave that has been reflected or otherwise returned with sufficient magnitude and delay to be perceived. Note 1: Echoes are frequently measured in dB relative to the directly transmitted wave. Note 2: Echoes may be desirable (as in radar) or undesirable (as in telephone systems. ) 3. In computing, to print or display characters (a) as they are entered from an input device, (b) as instructions are executed, or (c) as retransmitted characters received from a remote terminal. 4. In computer graphics, the immediate notification of the current values provided by an input device to the operator at the display console.