hy·po·na·tre·mi·a (h??p?-n?-tr??m?-?)n. An abnormally low plasma concentration of sodium ions.[hypo- + New Latin natrium, sodium; see hypernatremia + -emia.]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.