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. 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 have been created before full alphabets that had letters for both consonants and vowels, like the Greek alphabet. The earliest known abjad in the world is the Phoenician alphabet. Since Afro-Asiatic languages have a feature where the root meaning of the word is found in the word's string of consonants, abjads are widely used for those languages. There are even languages that don't have consonant roots that use abjads, such as Persian and Urdu, which both use the Arabic alphabet.