An abjad (//, Arabic: أبجد; also abgad) is a writing system in which only consonants are represented, leaving vowel sounds to be inferred by the reader. This contrasts with other alphabets, which provide graphemes for both consonants and vowels. The term was introduced in 1990 by Peter T. Daniels. Other terms for the same concept include: partial phonemic script, segmentally linear defective phonographic script, consonantary, consonant writing, and consonantal alphabet.
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, d — to replace the more common terms "consonantary" and "consonantal alphabet", in describing the family of scripts classified as "West Semitic".