An abjad is a type of writing system where each symbol always or usually stands for a consonant, leaving the reader to supply the appropriate vowel. It is a term suggested by Peter T. Daniels to replace the common terms "consonantary", "consonantal alphabet" or "syllabary" to refer to the family of scripts called West Semitic. Abjad is thought to be based on the first letters (a, b, ǧ, d) found in all Semitic language such as Phoenician, Syriac, Hebrew, and Arabic. In Arabic, "A" (ʾAlif), "B" (Bāʾ), "Ǧ" (Ǧīm), "D" (Dāl) make the word "abjad" which means "alphabet". The modern Arabic word for "alphabet" and "abjad" is interchangeably either "abajadeyyah" or "alefbaaeyyah". The word "alphabet" in English has a source in Greek language in which the first two letters were "A" (alpha) and "B" (beta), hence "alphabeta" (in Spanish, "alfabeto", but also called "abecedario", from "a" "b" "c" "d"). In Hebrew the first two letters are "A" (Aleph), "B" (Bet) hence "alephbet." It is also used to enumerate a list in the same manner that "a, b, c, d" (etc.) are used in the English language.
The name "abjad" (ʾabǧad أبجد) is derived from pronouncing the first letters of the Arabic alphabet in order. The ordering (ʾabǧadī ) of Arabic letters used to match that of the older Hebrew, Phoenician and Semitic alphabets; ʾ b g d (read from right to left: أ ب ج د) or أبجد.
According to the formulations of Daniels, abjads differ from alphabets in that only consonants, not vowels, are represented among the basic graphemes. Abjads differ from abugidas, another category invented by Daniels, in that in abjads, the vowel sound is implied by phonology, and where vowel marks exist for the system, such as nikkud for Hebrew and ḥarakāt for Arabic, their use is optional and not the dominant (or literate) form. Abugidas mark the vowels (other than the "inherent" vowel) with a diacritic, a minor attachment to the letter, or a standalone glyph. Some abugidas use a special symbol to suppress the inherent vowel so that the consonant alone can be properly represented. In a syllabary, a grapheme denotes a complete syllable, that is, either a lone vowel sound or a combination of a vowel sound with one or more consonant sounds.