An abjad (/ˈæbdʒæd/,[1]Arabic: أبجد), also abgad,[2][3] is a writing system in which only consonants are represented, leaving the vowel sounds to be inferred by the reader. This contrasts with alphabets, which provide graphemes for both consonants and vowels. The term was introduced in 1990 by Peter T. Daniels.[4] Other terms for the same concept include partial phonemic script, segmentally linear defective phonographic script, consonantary, consonant writing, and consonantal alphabet.[5]
Impure abjads represent vowels with either optional diacritics, a limited number of distinct vowel glyphs, or both.
^Daniels, P. (1990). Fundamentals of Grammatology. Journal of the American Oriental Society, 110(4), 727-731. doi:10.2307/602899: "We must recognize that the West Semitic scripts constitute a third fundamental type of script, the kind that denotes individual consonants only. It cannot be subsumed under either of the other terms. A suitable name for this type would be alephbeth, in honor of its Levantine origin, but this term seems too similar to alphabet to be practical; so I propose to call this type an "abjad," [Footnote: I.e., the alif-ba-jim order familiar from earlier Semitic alphabets, from which the modern order alif-ba-ta-tha is derived by placing together the letters with similar shapes and differing numbers of dots. The abjad is the order in which numerical values are assigned to the letters (as in Hebrew).] from the Arabic word for the traditional order6 of its script, which (unvocalized) of course falls in this category... There is yet a fourth fundamental type of script, a type recognized over forty years ago by James- Germain Fevrier, called by him the "neosyllabary" (1948, 330), and again by Fred Householder thirty years ago, who called it "pseudo-alphabet" (1959, 382). These are the scripts of Ethiopia and "greater India" that use a basic form for the specific syllable consonant + a particular vowel (in practice always the unmarked a) and modify it to denote the syllables with other vowels or with no vowel. Were it not for this existing term, I would propose maintaining the pattern by calling this type an "abugida," from the Ethiopian word for the auxiliary order of consonants in the signary."
^Amalia E. Gnanadesikan (2017) Towards a typology of phonemic scripts, Writing Systems Research, 9:1, 14-35, DOI: 10.1080/17586801.2017.1308239 "Daniels (1990, 1996a) proposes the name abjad for these scripts, and this term has gained considerable popularity. Other terms include partial phonemic script (Hill, 1967), segmentally linear defective phonographic script (Faber, 1992), consonantary (Trigger, 2004), consonant writing (Coulmas, 1989) and consonantal alphabet (Gnanadesikan, 2009; Healey, 1990). "