Accidental gap
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 that it would be permissible by the grammatical rules of that language. 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, for example, a word pronounced cannot exist because it  has no vowels and therefore does not obey the word-formation rules of English. This is a systematic gap. In contrast, a word pronounced would obey English word-formation rules, but this is not a word in English. Although theoretically such a word could exist, it does not; its absence is therefore an accidental gap, in the ontologic sense of the word "accidental" (that is, circumstantial rather than essential). 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. Phonological gaps. Often words that are allowed in the phonological system of a language are absent. For example, in English the consonant cluster is allowed at the beginning of words such as "spread" or "spring" and the syllable rime occurs in words such as "sick" or "flicker". Even so, there is no English  word pronounced *. Although this potential word is phonologically well-formed according to English phonotactics, 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 (). This lack of an expected distinction is commonly called a "hole in the pattern". 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 "-(t)ion" to certain verbs (typically words from Latin through Anglo-Norman French or Old French). Some verbs, such as "recite" have two related nouns, "recital" and "recitation". However, in many cases there is only one such noun, as illustrated in the chart below. Although in principle the morphological rules of English allow for other nouns, those words do not exist. Many potential words that could  be made following morphological rules of a language do not enter the lexicon. Blocking, including "homonymy blocking" and "synonymy blocking", stops some potential words. A homonym of an existing word may be blocked. For example, the word "liver" meaning "someone who lives" is only rarely used because the word "liver" (an internal organ) already exists. Likewise, a potential word can be blocked if it is a synonym of an existing word. An older, more common word blocks a potential synonym, known as "token-blocking". For example, the word "stealer" ("someone who steals") is also rarely used, because the word "thief  already exists. Not only individual words, but entire word formation processes may be blocked. For example, the suffix -ness" is used to form nouns from adjectives. This productive word-formation pattern blocks many potential nouns that could be formed with "-ity". Nouns such as *' (a potential synonym of "calmness") and *' (cf. "darkness") are unused potential words. This is known as "type-blocking". 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. *"derz'u" "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. The similar case of unpaired words occurs where one word is obsolete or rare while another word derived from it is more common. Examples include *"effable" and "ineffable" or *"kempt" and "unkempt". Semantic gaps. A gap in semantics occurs 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 common gender-neutral term for a parent's sibling, and traditionally none for a sibling's child. The separate words predicted on the basis of this semantic contrast are absent from the language, or at least from many speakers' dialects. It is possible to coin new ones (as happened with the word "nibling"), but whether those words gain widespread acceptance in general use, or remain neologistic  and resisted outside particular registers, is a matter of prevailing usage in each era.