Margin
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In telecommunication, the term margin has the following meanings:
1. In communications systems, the maximum degree of signal distortion that can be tolerated without affecting the restitution, i.e., without its being interpreted incorrectly by the decision circuit.
2. The allowable error rate, deviation from normal, or degradation of the performance of, a system or device.
Source: from Federal Standard 1037C
See also: profit margin