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.
When there's an opening or gap in a camera or optical equipment, stray light can enter the sealed interior, resulting in extra exposure on film or sensor. This additional light is diffused but may cause shadows or reflections within the device.