is a type of writing system
where there is one symbol per character (as in an alphabet
The term takes its name from the old order of the Arabic alphabet
Alif, Bá, Jim, Dál, though the word may have earlier roots in Phoenician
Abjads differ from alphabets in that they only have characters for consonantal sounds.
Some abjads (like the Arabic abjad) have characters for some vowels as well, but only use them in special contexts. All known abjads belong to the Semitic family of scripts, and derive from the original Northern Linear Abjad. The reason for this is that Semitic languages
have a morphemic structure
which makes the denotation of vowels
redundant in most cases.
Many scripts derived from abjads have been extended with vowel symbols to become full alphabets.
This has mostly happened when the script was adapted to a non-semitic language, the most famous case being the derivation of the Greek alphabet from the Phoenician abjad.
Other times, the vowel signs come in the form of little points or hooks attached to the consonant letters, producing an abugida.
See also Baha'i, where abjad is a numerological system.