TL;DR: Avoid work at height where possible. If you must, plan it, use the right equipment, and prevent falls—of people and objects.
Any place where a person could fall and be injured—above or below ground level. Includes ladders, scaffolds, MEWPs, fragile roofs, pits, and excavations.
Use suitable equipment for the task: checked ladders, towers built to spec, certified MEWPs, harnesses with compatible anchor points and rescue plans.