Diagram of specular reflection
Specular reflection is the perfect, mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected onto a single outgoing direction. Such behaviour is described by the law of reflection, which states that the direction of outgoing reflected light and the direction of incoming light make the same angle with respect to the surface normal; this is commonly stated as θi = θr.
This is in contrast to diffuse reflection, where incoming light is reflected in all directions equally. The most familiar example of the distinction between specular and diffuse reflection would be matte and glossy paints. Matte paints have a higher proportion of diffuse reflection, while glossy paints have a greater proportion of specular reflection.
Specular reflection and diffuse reflection are simply approximations. In reality, surfaces exhibit a continuum of modes of reflection between these two.