Y chromosome or Y-chro·mo·some (w??kr??m?-s?m?)n. The sex chromosome associated with male characteristics in mammals, not occurring in females and occurring with one X chromosome in the male sex-chromosome pair.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.Y chromosome n. a sex chromosome of humans and most mammals that is present only in males and is paired with an Xchromosome. Compare X chromosome. [1920?25] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.