A father-in-law is the father of someone's husband or wife. When two people get married, they gain a whole new set of family relationships. Your spouse's father becomes your father-in-law, while you become his son-in-law or daughter-in-law.

This relationship can feel a bit awkward at first, since unlike your own father, your father-in-law is someone you meet later in life and have to build a relationship with from scratch. Some people become extremely close with their fathers-in-law, treating them almost like their own parents. Others maintain a more formal, polite relationship.

You'll hear similar terms for other family members: mother-in-law, sister-in-law, brother-in-law.