A secondary significant other is any partner involved in a relationship with a polyamorous person who already has a primary partner. A secondary significant other is generally aware that the primary partnership takes priority over their relationship with their partner. Some people who use this language find that it helps them maintain defined boundaries in their relationships.
More About Secondary Significant Other
Terms like secondary significant other are not used by all polyamorous people. Some feel that this kind of language serves to designate one partner as "lesser" and creates a structure in which some people's needs get met and others do not.
This language can be used in a manner that is descriptive, such as simply explaining the nature of a relationship ("I only see Stacy occasionally, she is my secondary significant other.") or prescriptive which dictates appropriate behavior in that relationship ("Tom is my primary partner and Stacy is my secondary significant other. So, I should put Tom's needs first!").