What is Residence?
A residence consists of one or more dwelling units or rooming units, and any common areas, including one-family and two-family houses, multifamily dwellings or apartment hotels.
A two-family residence* is a building on a zoning lot containing two dwelling units occupied by two households. In R3-1, R3A, R3X, R4-1 and R4A districts, two-family houses, both detached and semi-detached, must have at least 75% of one dwelling unit directly above or below the other.
reference: Glossary of Planning Terms – NYC Planning