An abjad (/ˈæbdʒæd/,^[1] Arabic: أبجد), also abgad,^[2][3] is a writing system in which only consonants are represented, leaving 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]

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Impure abjads represent vowels with either optional diacritics, a limited number of distinct vowel glyphs, or both.

The name abjad is based on the Arabic alphabet's first (in its original order) four letters—corresponding to a, b, j, and d—to replace the more common terms "consonantary" and "consonantal alphabet", in describing the family of scripts classified as "West Semitic". Similar to other Semitic languages such as Phoenician, Hebrew and Semitic proto-alphabets: specifically, aleph, bet, gimel, dalet.

According to the formulations of Peter T. Daniels,^[6] abjads differ from alphabets in that only consonants, not vowels, are represented among the basic graphemes. Abjads differ from abugidas, another category defined 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 all vowels (other than the "inherent" vowel) with a diacritic, a minor attachment to the letter, a standalone glyph, or (in Canadian Aboriginal syllabics) by rotation of the letter. 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.

The contrast of abjad versus alphabet has been rejected by other scholars because abjad is also used as a term for the Arabic numeral system. Also, it may be taken as suggesting that consonantal alphabets, in contrast to e.g. the Greek alphabet, were not yet true alphabets.^[7] Florian Coulmas, a critic of Daniels and of the abjad terminology, argues that this terminology can confuse alphabets with "transcription systems", and that there is no reason to relegate the Hebrew, Aramaic or Phoenician alphabets to second-class status as an "incomplete alphabet".^[8] However, Daniels's terminology has found acceptance in the linguistic community.^[9][10][11]

The first abjad to gain widespread usage was the Phoenician abjad. Unlike other contemporary scripts, such as cuneiform and Egyptian hieroglyphs, the Phoenician script consisted of only a few dozen symbols. This made the script easy to learn, and seafaring Phoenician merchants took the script throughout the then-known world.

The Phoenician abjad was a radical simplification of phonetic writing, since hieroglyphics required the writer to pick a hieroglyph starting with the same sound that the writer wanted to write in order to write phonetically, much as man'yōgana (kanji used solely for phonetic use) was used to represent Japanese phonetically before the invention of kana.

Phoenician gave rise to a number of new writing systems, including the widely used Aramaic abjad and the Greek alphabet. The Greek alphabet evolved into the modern western alphabets, such as Latin and Cyrillic, while Aramaic became the ancestor of many modern abjads and abugidas of Asia.

Impure abjads have characters for some vowels, optional vowel diacritics, or both. The term pure abjad refers to scripts entirely lacking in vowel indicators.^[12] However, most modern abjads, such as Arabic, Hebrew, Aramaic, and Pahlavi, are "impure" abjads – that is, they also contain symbols for some of the vowel phonemes, although the said non-diacritic vowel letters are also used to write certain consonants, particularly approximants that sound similar to long vowels. A "pure" abjad is exemplified (perhaps) by very early forms of ancient Phoenician, though at some point (at least by the 9th century BC) it and most of the contemporary Semitic abjads had begun to overload a few of the consonant symbols with a secondary function as vowel markers, called matres lectionis.^[13] This practice was at first rare and limited in scope but became increasingly common and more developed in later times.

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The abjad form of writing is well-adapted to the morphological structure of the Semitic languages it was developed to write. This is because words in Semitic languages are formed from a root consisting of (usually) three consonants, the vowels being used to indicate inflectional or derived forms. For instance, according to Classical Arabic and Modern Standard Arabic, from the Arabic root ك‌ت‌ب K-T-B (to write) can be derived the forms كَتَبَ kataba (he wrote), كَتَبْتَ katabta (you (masculine singular) wrote), يَكْتُبُ⁩ yaktubu (he writes), and مَكْتَبَة⁩ maktabah (library). In most cases, the absence of full glyphs for vowels makes the common root clearer, allowing readers to guess the meaning of unfamiliar words from familiar roots (especially in conjunction with context clues) and improving word recognition^{[citation needed][dubious – discuss]} while reading for practiced readers.

By contrast, the Arabic and Hebrew scripts sometimes perform the role of true alphabets rather than abjads when used to write certain Indo-European languages, including Kurdish, Bosnian, Yiddish, and some Romance languages such as Mozarabic, Aragonese, Portuguese, Spanish and Ladino.

• Abjad numerals (Arabic alphanumeric code)
• Abugida
• Gematria (Hebrew & English system of alphanumeric code)
• Numerology
• Shorthand (constructed writing systems that are structurally abjads)