archiphoneme

an abstract phonological unit consisting of the distinctive features common to two phonemes that differ only in that one has a distinctive feature lacking in the other. The archiphoneme is said to be realized when in a certain position an otherwise phonemic opposition is neutralized; thus, in German, while p and b are separate phonemes differing only in the distinctive feature of voicing, in final position the voicing or unvoicing of the labial stop is nondistinctive, and the p- sound of leib “body” may be called the realization of the archiphoneme.

such a unit occurring in a position where the contrast between two or more phonemes is neutralized.