Related terms

Ménage à trois

A love triangle should not be confused with a ménage à trois, a three-way relationship in which all members are romantically involved with each other instead of being in conflict over one person.

Love rectangle

Love rectangle is a somewhat facetious term to describe a romantic relationship that involves four people, analogous to the typically three-sided love triangle. Many people use this term for a romantic relationship between two people that is complicated by the romantic attentions of two other people, but it is more frequently reserved for relationships where there are more connections. Minimally, both male characters usually have some current or past association with both female characters. These relationships need not be sexual; they can be friendships or familial relations. Both males and/or both females can also be friends, family members (frequently siblings) or sworn enemies.
Love rectangles tend to be more complicated than love triangles, often using their tangled relationships as a source of comedic humor.
An example of a love rectangle in classic literature is in William Shakespeare's A Midsummer Night's Dream, between the characters Lysander, Demetrius, Helena, and Hermia. Demetrius is granted Hermia's hand in marriage by her father, but Hermia loves Lysander, and the two flee, intending to elope. Demetrius pursues the couple, and Helena pursues Demetrius, whom she has always loved. The fairy Puck, in trying to use magic to resolve the situation, temporarily transfers both men's affections to Helena. Further tampering restores Lysander's love for Hermia. Demetrius, now in love with Helena, withdraws his claim on Hermia, and both couples are wed.
Another love rectangle happens in Mozart's Così Fan Tutte, where female characters Dorabella and Flordiligli (siblings) are Ferrando and Gugliemo's girlfriends respectively, and by the end of the opera they "accidentally" swap their boyfriends.
The love rectangle concept is popular in television programs such as Lost (Jack/Kate/Sawyer/Juliet), Glee (Finn/Rachel/Puck/Quinn), That '70s Show (Kelso/Jackie/Hyde/Laurie) and One Tree Hill (Lucas/Peyton/Nathan/Brooke). Also in the American drama film Closer(Alice/Dan/Anna/Larry).
For additional terms, the word "love" can be added to the front of other shapes to reflect romantic relationships involving more people, e.g. "love pentagon" or a "love hexagon."