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1. 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. (188) 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.