1. A setback, sometimes called step-back, is a step-like recession in a wall. Setbacks were initially used for structural reasons, but now are often mandated by land use codes, or are used for aesthetic reasons. In densely built-up areas, setbacks also help get more daylight and fresh air to the street level. Importantly, a setback helps lower the building's center of mass, making it more stable.
2. In land use, a setback is the minimum distance which a building or other structure must be set back from a street or road, a river or other stream, a shore or flood plain, or any other place which is deemed to need protection. Depending on the jurisdiction, other things like fences, landscaping, septic tanks, and various potential hazards or nuisances might be regulated and prohibited by setback lines. Setbacks along state, provincial, or federal highways may also be set in the laws of the state or province, or the federal government. Local governments create setbacks through ordinances, zoning restrictions, and Building Codes, usually for reasons of public policy such as safety, privacy, and environmental protection. Neighborhood developers may create setback lines (usually defined in Covenants & Restrictions and set forth in official neighborhood maps) to ensure uniform appearance in the neighborhood and prevent houses from crowding adjacent structures or streets. In some cases, building ahead of a setback line may be permitted through special approval.

Description sourced from Wikipedia. Images sourced from Wikipedia and Summit County.