Triliteral root

The roots of verbs and most nouns in the Semitic languages are characterized as a sequence of consonants or "" (hence the term consonantal root). Such abstract consonantal roots are used in the formation of actual words by adding the vowels and non-root consonants (or "transfixes") which go with a particular morphological category around the root consonants, in an appropriate way, generally following specific patterns. It is a peculiarity of Semitic linguistics that a large majority of these consonantal roots are triliterals (although there are a number of quadriliterals, and in some languages also biliterals).

A triliteral or triconsonantal root (Hebrew: שרש תלת-עצורי‎, šoreš təlat-ʻiṣuri; Arabic: جذر ثلاثي‎‎, jiḏr ṯulāṯī; Syriac: ܫܪܫܐ‎, šeršā) is a root containing a sequence of three consonants.

The following are some of the forms which can be derived from the triconsonantal root k-t-b (general overall meaning "to write") in Hebrew and Arabic:

Note: The Hebrew fricatives transcribed as "ḵ" and "ḇ" can also be transcribed in a number of other ways, such as "ch" and "v" , which are pronounced [χ] and [v], respectively. They are transliterated "ḵ" and "ḇ" on this page to retain the connection with the pure consonantal root k-t-b. Also notice that in Modern Hebrew, there is no gemination.

...
Wikipedia