Definition - What does Co-Spouse mean?
A co-spouse is a person in a group marriage who shares a spouse with another person. For example, someone may say, "My wife's other husband is a co-spouse." A female co-spouse is a co-wife, and a male co-spouse is a co-husband.
Kinkly explains Co-Spouse
Although group marriage is not legally recognized in North America, many polyamorous groups can develop a relationship with the same duties and responsibilities as marriage: bills, house cleaning and maintenance, child rearing, etc. They live together as a polycule and consider themselves married, despite the inability to do so under the law.
In these kinds of relationships, the members will often draw up civil and/or business contracts to define financial responsibilities and protections for each member. These contracts are not a marriage per se, but are legally binding. Co-spouses become just as responsible for each others' well-being as monogamous couples.