It is the dropping away of a flake of magnetic material from magnetic tape, leading to loss of signal, or a failure to properly read a binary character from data storage. In magnetic disk, tape, card, or drum systems, a dropout is a recorded signal with an amplitude less than a predetermined percentage of a reference signal. On an optical disc, it is an area of reduced or obscured reflectivity due to fingerprints, scratches, or other damage. It may also be a potential difference between the input and output terminals of a linear regulator, below which the regulator can not meet its specifications.
A dropout is also a momentary loss of signal in a communications system, usually caused by noise, propagation anomalies, or system malfunctions. For analog signals, a dropout is frequently gradual and partial, depending on the cause. For digital signals, dropouts are more pronounced, usually being sudden and complete, due to the cliff effect. In mobile telephony, a dropout of more than a few seconds will result in a dropped call.