an emotional relationship in which there are conflicts involving a man and two women or a woman and two men
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
A relationship involving three lovers, such as two women involved with one man or two men with one woman. For example, The plot of the murder mystery revolved around the eternal triangle of a husband, wife, and another woman. [c. 1900]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.