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Most significant other is a term that is sometimes used in the context of a hierarchical polyamorous relationship to refer to one's primary partner. The term indicates that while one may have several partners who they identify as their "significant others," this specific partner holds a place of greater importance or priority.
This term is not exclusive to the the polyamorous community. The idea of one person holding a position of primary importance in another's life can be understood by many people. Another term that may be used is primary relationship.