theirpossessive case of they: It is their book. Not to be confused with:there ? in or at that place: The book is over there. they’re ? they are: They?re working on a second edition.Abused, Confused, & Misused Words by Mary Embree Copyright © 2007, 2013 by Mary Embreetheir (thâr)adj.The possessive form of they1. Used as a modifier before a noun: their accomplishments; their home town.2. Usage Problem His, her, or its: “It is fatal for anyone who writes to think of their sex” (Virginia Woolf). See Usage Notes at he1, they.[Middle English, from Old Norse theira, theirs; see to- in Indo-European roots.]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.their (ð??) determiner1. of, belonging to, or associated in some way with them: their finest hour; their own clothes; she tried to combat their mocking her. 2. belonging to or associated in some way with people in general not including the speaker or people addressed: in many countries they wash their clothes in the river. 3. belonging to or associated in some way with an indefinite antecedent such as one, whoever, or anybody: everyone should bring their own lunch. [C12: from Old Norse theira (genitive plural); see they, them]Usage: See at theyCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014their (ð??r; unstressed ð?r) pron. 1. a form of the possessive case of they used as an attributive adjective, before a noun: their home; their rights as citizens. 2. (used after an indefinite singular antecedent in place of the definite form his or her): Someone left their book on the table. Compare theirs. [1150?1200; Middle English