Polite term for one's spouse's father. Used both when addressing him directly and when referring to him in conversation.
I hear my father-in-law is retiring next month.
Father-in-law, would you like some tea?
Can refer to a foster father in a family context, though 養父 (ようふ) is more common for legal foster relationships.
Used for a stepfather, especially in direct address or when speaking politely about him. 継父 (ままちち/けいふ) is a more formal or legal term.
He was raised by his foster father from a young age.
I go fishing with my stepfather on weekends.
Combination of the honorific prefix お, 義父 (gifu, 'father-in-law' or 'stepfather'), and the polite suffix さん. The reading おとうさん is a gikun (meaning-based reading) that mirrors the common word for 'father' (お父さん), reflecting the respectful treatment of a father-in-law as one's own father.