1. astute - Keen in discernment.
2. atheism - The denial of the existence of God.
3. athirst - Wanting water.
4. athwart - From side to side.
5. atomizer - An apparatus for reducing a liquid to a fine spray, as for disinfection, inhalation, etc.
6. atone - To make amends for.
7. atonement - Amends, reparation, or expiation made from wrong or injury.
8. atrocious - Outrageously or wantonly wicked, criminal, vile, or cruel.
9. atrocity - Great cruelty or reckless wickedness.
10. attache - A subordinate member of a diplomatic embassy.
11. attest - To certify as accurate, genuine, or true.
12. attorney-general - The chief law-officer of a government.
13. auburn - Reddish-brown, said usually of the hair.
14. audacious - Fearless.
15. audible - Loud enough to be heard.