A brother-in-law is an individual connected through marriage or a civil union with respect to one of the partners. For example, if a person is married, their spouse's brother becomes their brother-in-law. Similarly, if someone is married, their spouse's sister is considered their sister-in-law. Although there is no direct blood relationship, the brother-in-law is part of the family through conjugal ties or partner of one of the brothers or sisters.

A brother-in-law is sometimes like a brother.