A supermini is a stock 84cc mini (usually one of the five major dirtbike brands) with a modified cylinder of 100, 103, 105, 109, or 112cc's. These dirtbikes also have a 16" rear wheel and a 19" front wheel as opposed to the normal 14" and 17" wheels on a stock 85cc bike. These bike's ate the perfect transition bike from a mini to a full sized bike. Often, a skilled rider on a supermini can catch and pass an adult on a full sized bike.
"dude did you watch number 111 on the Yamaha supermini dirtbike?"
"ya, he was doin the 90 foot triple jump that most of big bikes won't even do!"