1.
husband
An informal, somewhat old-fashioned way to refer to one's husband. Often carries a traditional connotation of the husband as head of the household.
2.
master, host, proprietor
The master or proprietor of an establishment such as an inn or a tea ceremony host. Also used for the host of a tea gathering.
Usage
As "husband," 亭主 sounds somewhat old-fashioned and informal. It is typically used by wives referring to their own husbands in casual speech. Younger speakers tend to prefer 夫 or 旦那.
The compound 亭主関白 (lit. "husband regent") describes a domineering husband who expects to be waited on.
As "master/host," 亭主 is used in tea ceremony (茶道) to refer to the person hosting and serving tea.