Drift

In telecommunication, a drift is a comparatively long-term change in an attribute or value of a system or equipment operational parameter.

Note 1: The drift should be characterized, such as "diurnal frequency drift" and "output level drift."

Note 2: Drift is usually undesirable and unidirectional, but may be bidirectional, cyclic, or of such long-term duration and low excursion rate as to be negligible.

Source: from Federal Standard 1037C and from MIL-STD-188