Final answer:
A single dwelling unit spread over two floors connected by an indoor staircase is known as a duplex. This type of housing accommodates two families within the same structure but with separate living spaces.
Step-by-step explanation:
A single dwelling unit spread over two floors, which is connected by an indoor staircase, is referred to as a duplex. A duplex is typically a single residential structure divided into two units with separate entrances for each unit. The term 'duplex' can also refer to two units with a common wall or floor over two levels. The distinguishing feature of a duplex is that it is designed for two families, with each having its own living spaces and amenities such as kitchens and bathrooms, but they are contained within the same building structure.