PHONOLOGY: the theory that underlying representations are not fully
specified i.c. that predictable information is not underlyingly present.
EXAMPLE: in English there is no lexical distinction between aspirated
and non-aspirated stops. Still there is a phonetic difference between the [p^h]
in [p^h]in and the non-aspirated [p] in s[p]in.
Underspecification theory expresses this by assuming that underlyingly both p's are not specified for aspiration. The aspiration feature is later (post-lexically) specified by a context-sensitive rule inserting [+spread glottis] at the beginning of a syllable; the non-aspiration is a consequence of a universal rule which inserts [-spread glottis] in all other contexts.
See Structure preservation,