In linguistics an accidental gap, also known as a gap, paradigm gap, accidental lexical gap, lexical gap, lacuna, or hole in the pattern, is a potential word, word sense, morpheme, or other form that does not exist in some language despite being theoretically permissible by the grammatical rules of that language.[1] For example, a word pronounced /zeɪ̯k/ is theoretically possible in English, as it would obey English word-formation rules, but does not currently exist. Its absence is therefore an accidental gap, in the ontologic sense of the word accidental (that is, circumstantial rather than essential).
Accidental gaps differ from systematic gaps, those words or other forms which do not exist in a language due to the boundaries set by phonological, morphological, and other rules of that specific language. In English, a word pronounced /pfnk/ does not and cannot exist because it has no vowels and therefore does not obey the word-formation rules of English. This is a systematic, rather than 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 nonexistent words or word senses potentially allowed by the morphological system. A semantic gap refers to the nonexistence of a word or word sense to describe a difference in meaning seen in other sets of words within the language.