Gobos
"Gobo" is a common term that's used to describe an object that's used to break up the lighting, if you want to either avoid a light that's too even, or add some sort of shadow pattern to the light being projected.
There is also the "cucoloris" or "cookie" for short, in the real lighting world for film and stage work, the gobo is usually a metal disc with hole patterns that are placed in front of a light, or into it, if it's a light designed to use gobos. The cucoloris is usually larger, like a flag, with holes cut out to break up the light. More on theatrical gobos here.
Examples of a few gobos.
In the 3D world, things are a little easier, as you can usually use any image (color or grayscale) to project an image from the light, giving an effect much like a slide projector. Gobos are most often used with spotlights, though they can sometimes be used with other light types as well.
Here are a couple examples of scenes where gobo images were used to cast a pattern:
Gobos being used to break up and control light: Left, Ryse: Son of Rome. Right, Halo 5: Guardians.
These can be used in a lot of areas where you want to break up the light so it's not so perfectly even. They can also be used to control the shape of the light:
These are a few gobo images I've used. The one on the right was used as a "cave-hole" to let sunlight in. An image like this, although simple, will always look more realistic than a standard, round spotlight. Another nice thing about using gobos is that they can be used with lights that aren't casting shadows. The image being projected fakes the shadow, so all kinds of trickery can be used with them.
