From Longman Dictionary of Contemporary Englishlove trianglelove trianglea situation in which someone is having a sexual relationship with the partner of a close friend – used especially in newspapers → love
Examples from the Corpus
love triangle• All that, and there's still the Baldwin love triangle and Jim and Gwen's bust-up to come.• Alex and Thrash in bizarre love triangle!• There is nothing remarkable about the plot, which revolves around the classic love triangle. 