A polite term for someone else's husband (or the master/head of a household). Generally used respectfully; speakers usually do not use it for their own husband.
ご主人はお仕事で今お留守ですか。
Is your husband away for work right now?
あちらにいるのがご主人です。
The person over there is her husband.
ご主人のご都合の良い日にお伺いします。
I'll visit on a day that is convenient for your husband.