Simply put, a metamour is your partners other partner. For example, my girlfriends husband is my metamour.
Metamour: A person who is in an intimate (romantic or sexual) relationship with an intimate partner of yours. In poly and open relationships, the people involved typically all know each other — at least, for relationships that have been going on for a while and have become emotionally invested.
What is a nesting partner?
A nesting partner, on the other hand, is a live-in partner (or partners). This person may or may not be a primary partner as well, but “nesting partner” is often used to replace the term primary partner, while still describing a higher level of entanglement, in order to avoid hierarchical language.