aq·ui·tard (?k?w?-tärd?, ä?kw?-)n. A body of rock or stratum of sediment that retards but does not prevent the flow of groundwater from one aquifer to another.[aqui- + (re)tard.]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.