The term "opaque" refers to a material or object that does not allow light to pass through it. This means that when light hits an opaque surface, it cannot be seen through, making it impossible to see what is on the other side. Common examples of opaque materials include wood, metal, and stone. In contrast to transparent materials like glass or water, which allow light to pass through, opaque objects absorb or reflect light. This property is important in various applications, such as in building design, where opaque walls provide privacy and protection from the outside environment.