In linguistics an accidental gap, also known as a gap or a hole in the pattern, is a word or other form that does not exist in some language but which would be expected to exist given the grammatical rules of the language. For example, in English a noun may be formed by adding the suffix -al to a verb (e.g. recite → recital; arrive → arrival), yet there is no English word describal related to the verb describe. Although theoretically such a word could exist, it does not; its absence is therefore an accidental gap.
Various types of accidental gaps exist. Phonological gaps are either words allowed by the phonological system of a language which do not actually exist, or sound contrasts missing from one paradigm of the phonological system itself. Morphological gaps are non-existent words predicted by the morphological system, such as describal mentioned above. A semantic gap refers to the non-existence of a word to describe a difference in meaning seen in other sets of words within the language.
 Phonological gaps
Often words that are allowed in the phonological system of a language are absent. For example, in English the consonant cluster /bl/ is allowed at the beginning of words such as blind or blister and the syllable rime /ɪk/ occurs in words such as sick or flicker. Even so, there is no English word pronounced */blɪk/. Although this potential word is phonologically well-formed, it happens to not exist.
The term "phonological gap" is also used to refer to the absence of a phonemic contrast in part of the phonological system. For example, Thai has several sets of stop consonants that differ in terms of voicing (whether or not the vocal cords vibrate) and aspiration (whether a puff of air is released). Yet the language has no voiced velar consonant (/ɡ/). This lack of an expected distinction is commonly called a "hole in the pattern".
|plain voiceless||aspirated voiceless||voiced consonant|
 Morphological gaps
A morphological gap is the absence of a word that could exist given the morphological rules of a language, including its affixes. For example, in English a deverbal noun can be formed by adding either the suffix -al or -tion to a verb. Some nouns of this pattern simply do not exist, even though there is no grammatical reason for them not to.
|verb||noun (-al)||noun (-tion)|
A defective verb is a verb that lacks some grammatical conjugation. For example, several verbs in Russian do not have a first-person singular form in non-past tense. Although most verbs have such a form (e.g. vožu "I lead"), about 100 verbs in the second conjugation pattern (e.g. *deržu "I talk rudely"; the asterisk indicates ungrammaticality) do not appear as first-person singular in the present-future tense. Morris Halle called this defective verb paradigm an example of an accidental gap.
 Semantic gaps
In semantics a gap may be noted when a particular meaning distinction visible elsewhere in the lexicon is absent. For example, English words describing family members generally show gender distinction. Yet the English word cousin can refer to either a male or female cousin. Similarly, while there are general terms for siblings and parents, there is no comparable gender-neutral term for an aunt or uncle. The separate words predicted on the basis of this semantic contrast are absent from the language.
 See also
- Semantic gap in computer programming languages and natural language processing
- Crystal, David (2003). A Dictionary of Linguistics and Phonetics. Malden: Wiley-Blackwell. ISBN 0-6312-2664-8.
- Halle, Morris (1973). "Prolegomena to a theory of word-formation". Linguistic Inquiry 4: 451–464.
- Trask, Robert Lawrence (1996). A Dictionary of Phonetics and Phonology. London: Routledge.
- Abramson, Arthur S. (1962). The Vowels and Tones of Standard Thai: Acoustical Measurements and Experiments. Bloomington: Indiana University Research Center in Anthropology, Folklore, and Linguistics.
- Kerstens, Johan, Eddy Ruys and Joost Zwarts, ed. (2001). "Accidental gap". Lexicon of Linguistics. Utrecht institute of Linguistics OTS. Retrieved 2011-02-12.
- Quinion, Michael (23 November 1996). "Unpaired words". World Wide Words. Retrieved 31-07-2012.