When trying to define what a bike is there are so many possibilities. Of course it has two wheels and some way of steering, some means of propulsion by your own power and something to sit or perch on. But I wouldn't want to discriminate against trikes (for example the excellent and adaptable Kettweisel that I reviewed for Cycle earlier this year, or the four wheeled machines used by people with disabilities).
So how would I best define the bike?
Simple, for me a bike is:
"A smile on wheels"