A light leak is a hole or gap in the body of a camera, or other optical instrument, where light is able to "leak" into the normally light-tight chamber, exposing the film or sensor with extra light. This light is diffuse, although parts within the camera may cast shadows or reflect it in a particular way.
A light leak occurs when there's an opening or gap in a camera or optical device that allows stray light to enter the sealed interior, causing exposure on film or sensor with additional illumination. This extra light is diffused but can create shadows or reflections within the instrument.