What term is used to describe the act of impairing the appearance of something?

The term that describes the act of impairing the appearance of something is "mar." To mar something means to spoil or damage its surface or quality, thus affecting the aesthetic appeal negatively. This term is often used in contexts where an object is scratched, stained, or otherwise imperfect, which detracts from its original condition or beauty.

In contrast, the other terms relate to improving or adding to appearance. "Enhance" refers to increasing the quality, value, or attractiveness. "Beautify" is the act of making something more beautiful, and "adorn" means to add decoration to something, making it more visually appealing. Each of these alternatives signifies a positive alteration rather than impairment, which highlights why "mar" is the correct choice for describing a detrimental effect on appearance.