Roughness

The roughness parameter controls the roughness of the surface, with lower roughnesses corresponding to a smoother, more polished surface with mirror-like reflections.

The roughness varies from zero to one, and it can be set using a texture or a shader.

Example of Phong materials with different roughnesses.

Materials which have a roughness attribute:
Phong
Glossy Transparent
Coating
Double-sided Thin