What is a metamour?
A metamour is your partner's other partner — someone with whom you share a partner but with whom you are not romantically or sexually involved yourself. The term is most common in polyamorous communities. Metamour relationships range from close friendship — known as "kitchen table polyamory" because everyone is comfortable enough to share a meal — to deliberate non-contact, called "parallel polyamory". Both styles work; both are accepted; the choice is up to the people involved.