A- or a- may refer to:

A-hyphen

• A- (plane), a U.S. military aircraft prefix
• Privative a, a prefix expressing negation
• Copulative a, a prefix expressing unification

A-minus

• A−, a blood type in the ABO blood group system
• A− (grade), an educational grade in the letter-grading system, above B+ but below A

• Ā