As nouns the difference between abjad and sukun

is that abjad is a writing system, similar to a syllabary, in which there is one glyph (that is a symbol or letter) for each consonant or consonantal phoneme. Some languages that use abjads are Arabic, Hebrew, Persian, and Urdu. Abjads differ from syllabaries (such as the Japanese hiragana) in that the vowel quality of each letter is left unspecified, and must be inferred from context and grammar while sukun is a diacritic (ـْ) used in the Arabic abjad to mark the absence of a vowel.