Absolute neutralization

In phonology, absolute neutralization is the phenomenon that a segment of the underlying representation of a morpheme is not realized in any of its phonetic representations.

Example
Chomsky & Halle (1968) assume that the underlying representation of the word ellipse contains a final segment /e/ even though this segment is never pronounced. But the assumption of this segment in underlying representation explains the exceptional stress pattern of the word, i.e. that of trisyllabic words instead of that of bisyllabic words, i.e. /ellípse/ instead of /éllipse/. The segment /e/ is deleted after the assignment of stress: thus the opposition between /e/ and zero (the absence of a segment) is neutralized (see also: contextual neutralization).

Link
Utrecht Lexicon of Linguistics

Other languages
German absolute Neutralisierung