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.
She shared the apartment with her live-in partner.
她和同居伴侣一起住在这间公寓里。
A live-in servant or other domestic worker sleeps and eats in the house where they work.
I have a live-in nanny for my youngest daughter.
我为我小女儿雇了一个住家保姆。