In the terminology used to discuss the grammar of the Semitic languages, a quadriliteral is a consonantal root containing a sequence of four consonants (instead of three consonants, as is more often the case). A quadriliteral form is a word derived from such a four-consonant root. For example, the abstract quadriliteral root t-r-g-m / t-r-j-m gives rise to the verb forms תרגם tirgem in Hebrew and ترجم tarjama in Arabic, meaning "he translated". In some cases, a quadriliteral root is actually a reduplication of a two-consonant sequence. So in Hebrew דגדג digdeg means "he tickled", and in Arabic زلزال zilzāl means "earthquake".

Generally, only a subset of the verb derivations formed from triliteral roots are allowed with quadriliteral roots. For example, in Hebrew the Pi``el, Pu``al, and Hitpa``el, and in Arabic forms similar to the stem II and stem V forms of triliteral roots.

Traditionally in the Semitic languages, forms with more than four basic consonants (i.e. consonants not introduced by morphological inflection or derivation) were occasionally found in nouns — mainly loanwords from other languages — but never in verbs. However, in modern Israeli Hebrew, syllables are allowed to begin with a sequence of two consonants (a relaxation of the situation in early Semitic, where only one consonant was allowed), and this has opened the door to apparent five root-consonant forms, such as טלגרף tilgref "he telegraphed". But, -lgr- always appears as an indivisible cluster in the derivation of this verb, so that these five root-consonant forms do not display any fundamentally different morphological patterns from four root-consonant forms (and the hypothetical term "quinqueliteral" would be misleading if it implied otherwise).