Taken from the HUD manual...

Accessory Unit / Accessory Dwelling Unit

The accessory unit is defined as a habitable living unit added to, created within, or detached from a single-family dwelling that provides the basic requirements for living, sleeping, eating, cooking, and sanitation.

Accessory Dwelling Units (ADUs) are commonly understood to be a separate additional living unit, including separate kitchen, sleeping, and bathroom facilities, attached or detached from the primary residential unit, on a single-family lot.  ADUs are usually subordinate in size, location, and appearance to the primary unit and may or may not have separate means of ingress or egress.

Attached units, contained within a single-family home, known variously as "mother-in-law apartments," are the most common type of accessory dwelling unit.  Accessory units usually involve the renovation of a garage, basement, or small addition to a single-family home.