a·bac·a·vir (?-b?k??-vîr?)n. An antiviral drug, C14H18N6O, that is a nucleoside reverse transcriptor inhibitor and is used in its sulfate form in combination with other drugs for the treatment of HIV infection.[aba- (perhaps from ab, first two letters of the alphabet, from the fact that it was the first of this type of antiviral developed) + -c(arbocyclic nucleoside), nucleoside whose oxygen in the sugar ring has been replaced by a carbon + a(nti)vir(al) .]American Heritage® Dictionary of the English Language, Fifth Edition.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.