Which word best describes something that is impenetrable by light?

The term that best describes something that is impenetrable by light is "opaque." Opaque materials do not allow light to pass through them, which means they completely block any light from being transmitted. This characteristic is crucial in various contexts, such as materials used in construction, clothing, and optical devices.

In contrast, translucent materials allow some light to pass through but scatter it, meaning that objects cannot be clearly seen through them. Transparent materials, on the other hand, allow light to pass through with little to no obstruction, making objects on the other side clearly visible. The word "clear" also refers to the ability of a material to allow light to pass through without any obstruction or distortion, similar to transparent materials. Therefore, the distinct property of being impenetrable by light is precisely what makes "opaque" the most appropriate choice.