Key deer or key deern. A small subspecies of white-tailed deer (Odocoileus virginianus subsp. clavium), found only in the Florida Keys.[After the Florida Keys.]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.