Originally Posted by bhtooefr

Most jurisdictions define a bicycle as a human powered vehicle with 2 or 3 wheels.

However, there are sometimes some loopholes. Sometimes there's a wheel size requirement - usually larger than 16" - to qualify as a vehicle (so that children can ride their bicycle on a sidewalk, but hey, Greenspeed does 16" all around trikes).

That said, it's certainly not a good idea to ride a trike on the sidewalk if bicycles can't safely ride there as well.