A metamour is a word used within the polyamory community to describe one's partner's other partners, such as a husband's boyfriend, a girlfriend's lover, etc. The prefix "meta" comes from Greek, and means "after" or "beyond." "Amour" is the French for "love," or "lover" from the Latin "amor." The term is often abbreviated to "meta."
Metamours may meet and have a relationship, or they may not. In some cases, metamours want to know each other and get along; they may even become close friends or exist as a kind of family, as in kitchen table polyamory. In other relationships, people may choose not to mingle, as in parallel polyamory, or even adopt a don't-ask-don't-tell type of relationship when it comes to metamours.
In typical polyamorous relationships, metamours are part of the polycule, but not necessarily close to those with whom they are not having a more intimate romantic or sexual relationship. When a person starts dating their metamour, that person then becomes a partner or lover, and ceases to be a metamour.