A character whose primary role in the show is to relieve tension with oddball and/or hysterical, buffoonish antics. They have a job to do, but don't get into the thick of things and can stay somewhat detached. Since their purpose is to relieve tension, they almost always have immunity to dying when compared to the other characters. A guaranteed sign that the series is suffering from Cerebus Syndrome or that things are just going to go to Hell from here is to see whether this character either dies, leaves/is left behind or has a complete and utter meltdown, no matter how deserved.
This trope is also known as the "comic relief" and in The Team trope it is known as the Funny Guy. Within The Team they serve the same purpose as in the cast ensemble but this is specifically for the in-universe team members instead of the show's audience. If they can fight, they're likely to be a Fighting Clown.
The term comes from Sam Rockwell's "Guy Fleegman" character in the movie Galaxy Quest (who, in a non-genre blind example of Lampshade Hanging, is afraid that he is only a Red Shirt, until another character suggests that maybe he is instead the "plucky comic relief"), which of course is spoofing the television show Star Trek.
In some cases, similar to Stupid Boss. Many sidekicks fit this description, as well. See Amusing Alien for otherworldly characters whose only purpose is comedy. See The Face for where they have a purpose greater than comedy. Compare Cloudcuckoolander (whereas the humor comes out of sheer eccentricities rather than just being comical, and it can sometimes go hand in hand with each other).
