From The Unicode Standard 4.0 (Section 6.1 Writing Systems, page 148):

An abjad is a consonant writing system. The main letters are all consonants (or long vowels), with other vowels either left out entirely or indicated with secondary marking of the consonants. The best-known example of an abjad is the Arabic writing system. Indeed, the term "abjad" is derived from the first four letters of the traditional order of the Arabic script. Abjads are often, although not exclusively, associated with Semitic languages, which have word structures particularly well suited to the use of consonantal writing.

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