The word incoherent is typically used to describe a form of light in which the actual light waves are "scattered". An example of an incoherent light is the common desklamp. An example of coherent light is a laser beam.

The typical effects of light being coherent is that it stays the same diameter as the focal point, virtually forever.

Incoherent is also sometimes used to describe a moron, or an intoxicated person. This is usually indicated by a pattern of unrecognizeable speech.