The head of a household or the proprietor/owner of a shop or property (landlord/landlady).
彼はこの旅館の主人だ。
He is the proprietor of this inn.
店の主人に値段について尋ねた。
I asked the shop owner about the price.
アパートの主人が修理を手配してくれた。
The landlord arranged the repairs.
One's husband (used by a speaker to refer to their own husband; when referring to someone else's husband, people often say ご主人).
うちの主人は料理が上手です。
My husband is good at cooking.
彼女は主人の帰りを今か今かと待っている。
She is eagerly waiting for her husband's return.
私の主人は会社員です。
My husband is a company employee.
An employer or master (someone who has servants, apprentices, or employees under them).
彼は昔、陶芸家の主人のもとで修行した。
He once trained under a master potter.
召使は主人の命令に従わなければならなかった。
The servant had to obey his master’s orders.
A host or hostess (the person who entertains guests or runs an establishment and receives customers).
パーティーの主人が最後に挨拶をした。
The host gave a closing speech at the party.
料亭の主人が座敷に挨拶に回った。
The restaurant's owner went around greeting the guests in the tatami rooms.