in-group (?n?gro?op?)n. A group of people united by common beliefs, attitudes, or interests and usually excluding outsiders; a clique.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.in-group n (Sociology) sociol a highly cohesive and relatively closed social group characterized by the preferential treatment reserved for its members and the strength of loyalty between them. Compare out-group Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014in?-group` or in?group`, n. a group of people sharing similar interests, attitudes, etc., and usu. considering those outside the group as inferior or alien. Compare out-group. [1905?10] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.