[...] a feature F is "distinctive" in a language if and only if it appears as specified ([+F] or [-F]) in basic lexical entries. - Kiparsky (2004), a pag.176
[...] to say that a feature F is distinctive in a given context C is to say that the specification +F or -F occurs in C in the lexicon. Otherwise (i.e. if only 0F occurs), F is nondistinctive. - Kiparsky (2004), a pag.166