Real Dictionary

live in in American English
to reside at or away from the place of one's employment, esp. as a domestic servant
adjective: residing at the place of one's employment
noun: a live-in person

live in in British English
(of an employee, as in a hospital or hotel) to dwell at one's place of employment
living at the place where one is employed, esp. in domestic service
to live at the place where one is in domestic service
adjective: living in the place at which one works
adjective: of someone, such as an employee or a romantic partner, who lives in your home with you

live-in
A live-in partner is someone who lives in the same house as the person they are having a sexual relationship with, but is not married to them.