1A polite, honorific way to refer to someone else's husband (the listener's or a third party's). Not normally used for one's own husband.
ご主人はお元気でいらっしゃいますか。
How is your husband doing?
すみません、ご主人はご在宅でしょうか。
Excuse me, is your husband home?
田中さんのご主人は銀行にお勤めだそうです。
I hear Mrs. Tanaka's husband works at a bank.
あちらでお子さんと遊んでいらっしゃるのが、彼女のご主人です。
The person playing with the kids over there is her husband.