de·fi·cien·cy (d?-f?sh??n-s?)n. pl. de·fi·cien·cies 1. The quality or condition of being deficient; incompleteness or inadequacy.2. A lack or shortage, especially of something essential to health: a nutritional deficiency.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.deficiency (d??f???ns?) n, pl -cies1. the state or quality of being deficient2. a lack or insufficiency; shortage3. (Accounting & Book-keeping) another word for deficit4. (Genetics) biology the absence of a gene or a region of a chromosome normally presentCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014de?fi?cien?cy (d??f?? ?n si) n., pl. -cies. 1. the state of being deficient; lack; insufficiency. 2. the amount or quality lacked. [1625?35;