Cameo
A cameo is a brief appearance of an already known character, which is often non-speaking and in the background, and can either be a known Futurama character, a character from another series, or a real person. It is often included in a fitting scene (for example, the Gay Robot from Shhh...Don't Tell appearing in "Proposition Infinity"), but it can also just be a character walking by in the background (such as Nine, an easter egg having multiples crowd appearances before being given a major role in Into the Wild Green Yonder).
Image gallery
Nine on the right (2ACV11).
The audience scene (5ACV16).
