ac-pref. Variant of ad-..American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.AC or A.C. 1. air conditioning. 2. Also, ac, a.c., alternating current. 3. before Christ. [