A "rental unit" is a place or space that is leased or rented to someone for a specific period of time, typically for living or conducting business in. Full definition
The restriction in supply of rental units in an area also raises the price of owner - occupied homes, undermining their support against rent control. (mises.org)
When calculating rent estimates, researchers draw a sample from the complete set of rental units in a given area. (apartmentlist.com)
And, at least in my city, additional property taxes are charged for rental units on top of regular property taxes. (jamesaltucher.com)