I also take crcus trick as something which looks crazy, but is essentially useless to do anything with. Pressure flips never popped, and very few people legitimately catch late tricks. So certain tricks like kickflips which can basically be done onto into or over anything that can be ollied evolved, and others just stayed as stupid things dorks did on flatground only. Honestly, I think Laser flips might be crossing over. I don't think before the past couple of years anybody was doing laser flips on anything but flatground or maybe down a 3-4 stair, but now you got T-Pud doing them over gnarly handrails. Not sure I've ever seen it done into or out of any grind/slide yet though, whereas pretty much every other trick mentioned as "not a circus trick" has. People have done 360 flips into about everything, noselide, nosegrind, tailslide, lipslide, 5050, 5-0... laser, not so much
Also I'm not sure there is a dictionary definition of "circus trick" so really, its just what skateboarders as a society decide upon as a circus trick. There may not be a definition, but there are lots of similar traits to commonly agreed upon circus tricks.