Underlying representation

In phonology and morphophonology, an underlying representation (UR) or underlying form (UF) is a hypothesized, abstract representation of a morpheme or word stored in the lexicon and used as the input to phonological analysis. The output of phonological analysis is commonly described as a surface representation (SR), which reflects the effects of phonological processes and may differ systematically from an underlying form.

Source: Wikipedia — Underlying representation (CC BY-SA 4.0)