Can there be a Apartment rented out from a Single Family Home?

[Sunir Manandhar]

I am very New to the Industry and Need Help with words and terms used in Industry. 

Can there be a room rented out as an Apartment in a Single Family Home with multiple rooms? 

Isn't Apartment the term used for Unit? , then why can't we rent out a room in a single-family home and call it a single-bedroom apartment?