1. appreciable - Capable of being discerned by the senses or intellect.
2. apprehend - To make a prisoner of (a person) in the name of the law.
3. apprehensible - Capable of being conceived.
4. approbation - Sanction.
5. appropriate - Suitable for the purpose and circumstances.
6. aqueduct - A water-conduit, particularly one for supplying a community from a distance.
7. aqueous - Of, pertaining to, or containing water.
8. arbiter - One chosen or appointed, by mutual consent of parties in dispute, to decide matters.
9. arbitrary - Fixed or done capriciously.
10. arbitrate - To act or give judgment as umpire.
11. arbor - A tree.
12. arboreal - Of or pertaining to a tree or trees.
13. arborescent - Having the nature of a tree.
14. arboretum - A botanical garden or place devoted to the cultivation of trees or shrubs.
15. arboriculture - The cultivation of trees or shrubs.