A circus was a group of performing artists who put on shows involving exotic animals, stunts, humor, and curiosities.
In 2267, in an effort to goad Spock into an altercation, James T. Kirk told him he belonged in a circus, not on a starship, right next to the dog-faced boy. (TOS: "This Side of Paradise")
A circus could also be a metaphor for a chaotic, absurd series of events. Gillian Taylor was certain that there would be a media circus when George and Gracie were shipped out of the Cetacean Institute. (Star Trek IV: The Voyage Home)
While presiding over a hearing that would decide whether Quinn had a right to be euthanized, Kathryn Janeway warned Q that she would not have the proceedings turned into a circus. (VOY: "Death Wish")
B'Elanna Torres asked Seven of Nine if she remembered the way a particular Jefferies tube on USS Voyager had looked before she had turned it into a Borg circus. (VOY: "The Gift")
Citing Leonard and Roy as examples, The Doctor compared Reginald Barclay's workplace to a three-ring circus and said he ought to charge admission. (VOY: "Life Line")