California Code, Civil Code - CIV § 4125

(a) A “condominium project” means a real property development consisting of condominiums.

(b) A condominium consists of an undivided interest in common in a portion of real property coupled with a separate interest in space called a unit, the boundaries of which are described on a recorded final map, parcel map, or condominium plan in sufficient detail to locate all boundaries thereof. The area within these boundaries may be filled with air, earth, water, or fixtures, or any combination thereof, and need not be physically attached to land except by easements for access and, if necessary, support. The description of the unit may refer to (1) boundaries described in the recorded final map, parcel map, or condominium plan, (2) physical boundaries, either in existence, or to be constructed, such as walls, floors, and ceilings of a structure or any portion thereof, (3) an entire structure containing one or more units, or (4) any combination thereof.

(c) The portion or portions of the real property held in undivided interest may be all of the real property, except for the separate interests, or may include a particular three-dimensional portion thereof, the boundaries of which are described on a recorded final map, parcel map, or condominium plan. The area within these boundaries may be filled with air, earth, water, or fixtures, or any combination thereof, and need not be physically attached to land except by easements for access and, if necessary, support.

(d) An individual condominium within a condominium project may include, in addition, a separate interest in other portions of the real property.