Maricopa County tries to preserve the natural landscape by protecting some areas with steeper slopes. But “hillside” does not refer to any side of a hill. Here is a definition (Maricopa County Zoning Ordinance 1201):
All portions of a lot, tract or parcel having a natural slope of 15% or greater within any horizontal distance with a ten foot (10’) elevation change. Any challenge to a determination by the Department that any portion of a lot, tract or parcel is subject to the regulations set forth in this Section, shall include as part of the challenge a written determination of an Arizona registered civil engineer supported by sealed topographical plans.
A 15% slope is really just an elevation change of 1 foot per 6 2/3 feet horizontal distance.
Hillside does come with a few extra restrictions attached. Any portion of a property that is considered hillside by this definition and that is within the setbacks may not be disturbed other than for utilities or a driveway. General grading is not allowed!