An abjad is an alphabet in which all its letters are consonants. Even though vowels can be added in some abjads, they are not needed to write a word correctly. Abjad are commonly written from right to left (RTL), rather than left to right seen in other writing system. Well-known examples of abjads are the Arabic alphabet and the Hebrew alphabet.
Abjads are the first writing systems that were made only to show a word's pronunciation, instead of its meaning, unlike ideographs or ideograms, and they were created before full alphabets, like the Greek alphabet, which have letters for both consonants and vowels.
The earliest known abjad in the world is the Phoenician alphabet. Since in Afro-Asiatic languages, the root meaning of a word is found in its consonants, abjads are widely used in those languages. There are also languages without consonant roots that use abjads, such as Persian and Urdu, which both use the Arabic alphabet.