Shadows cast by objects in front of lights, whether raytraced, shadow maps, shadow volumes, or any other technique
A shadow is a dark region caused when light from an emitter is blocked by a shadowing object, causing a contrast between the dark region and the surrounding region which is fully lit. The emitter might be a point, cone, cylindrical light, an area light or image-based light. The shadowing object might be fully opaque or partially transparent, in which case it will cast coloured shadows (like stained glass). The dark region might be the lighting on another surface (the shadowed object), or it might be an environment shadow in some kind of volumetric medium.
This tag is appropriate whatever technique you're using to simulate shadows, whether by shadow rays in a raytracer, some kind of shadow maps or volumes, or something else completely.
If your problem is with shadowing indirect light in a global illumination system, you probably want a more specific tag. If your problem is with self-shadowing of micro-geometry, e.g. in the derivation of a shader, you should use a tag about that.
