A gobo describes a template that is attached to a light source to generate a design that displays on the floor, the ceiling or the wall.
Mongrams are popular for gobos used at weddings.
The word gobo is derived from the term ‘go between'.
Merriam-Webster Online Dictionarygobo (noun)
a dark strip (as of wallboard) to shield a motion-picture or television camera from light
a device to shield a microphone from sound