Certain words used in this act shall be defined, for the purposes thereof, as follows:
"Tenement house" is any house or building, or portion thereof, which is:
"Apartment" is a room or suite of two or more rooms, occupied or leased for occupation, or intended or designed to be occupied, as a family domicile.
"Basement" is a story partly, but not more than one-half,--"cellar" is a story more than one-half,--below the level of the street or ground adjacent to or adjoining the same; where the grade of a street adjacent to, or of ground surrounding, a tenement house varies, the mean or average grade of such street or ground shall be regarded as the grade of such street or ground, in the meaning of this act.
53 P.S. § 25017