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. For most purposes this is considered a problem. Within the lomography movement it is seen as a positive effect, giving photos character.
One frequent source of light leaks in 35 mm cameras is around the film door due to degrading foam. Replacing the foam is a simple matter. Medium format system cameras or large format cameras may have leaks between their various interchangeable parts or in old leather bellows. Electrical tape is often used to repair light leaks in these cases.
A light leak, considered as a problem, is a kind of stray light. It is possible to have a "virtual" light leak in spectral regions, like portions of the IR spectrum at room temperature, where surfaces inside the system emit significant amounts of radiation.