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1.
A gravitron is the ride at those crappy city to city carnivals that spins real fast. It smells of vomit and sweat and features weight lifting benches on rollers along the wall. Your back is placed upon the bench with you in a semi standing position and the gravitron is activated. Once it reaches speed the benches slide up giving you the feeling of weightlessness. In actuality it is more the centrifugal/centripedal force that allows you to "stick" to the wall.
This guy i work with plans to buy a gravitron and travel the country as a carnie, but we were looking on usedrides.com and they're like $40,000 used!
by blake April 01, 2004
 
2.
A "gravitron" is an all-glass gravity/vacuum pipe that is commonly used to smoke flavored tobacco and other legal substances.
I smoked out of a gravitron last night at my friend's place.
by Jon Evans May 22, 2006
 
3.
n. a water-differential pipe crafted from an empty milk jug; gravity bong.
"after taking three hits out of the gravitron, i spent the rest of the night curled up next to the toilet.
by UberGanz July 23, 2008