- Either of two angles formed by two intersecting lines and lying on opposite sides of the point of intersection.对顶角：由两条相交线形成的两个角，它们分别处在相交点的两边
- Aristotelian logic
- Aristotelians
- Aristotelic
- Aristotle
- arithmetic
- arithmetical
- arithmetic average
- arithmetic mean
- arithmetic progression
- arithmetics
- arithmetic sequence
- -arium
- -ariums
- Arius
- Arizona
- Arizona College of the Bible
- Arizona College of the Bible
- Arizona State University
- Arizona Western College
- Arjuna
- ark
- Arkansas
- Arkansas Baptist College
- Arkansas Baptist College
- Arkansas College