Accidental gap

An accidental gap is a non-existing word which is expected to exist given the hypothesized morphological rules of a particular language.

Examples
In English it is possible to derive nouns from verbs by adding the suffixes -al and -(a)tion to the verbal stem. However, some such derivations do not exist, although there are no grammatical reasons for their nonexistence. Compare the following examples:

(i) recite	 recital      recitation propose	 proposal     proposition (ii) arrive	 arrival     *arrivation refuse	 refusal     *refusation (iii) derive	 *derival     derivation describe  *describal    description

Comments
In the literature, an accidental gap is usually thought of as a hole in a paradigm.

Link
Utrecht Lexicon of Linguistics