How is waste oil defined in terms of its characteristics?

Waste oil is defined primarily by its degradation in quality and characteristics after being used. When oil significantly changes since it was originally refined, this alteration can result from factors such as contaminants, degradation due to heat and pressure, or the presence of heavy metals and other substances which make it unsuitable for continued use in its original application.

This transformation impacts the oil's physical and chemical properties, rendering it hazardous waste under various environmental regulations. Therefore, the characterization of waste oil is largely focused on how it has deteriorated from its initial state, which aligns with the definition provided in the correct answer.