Not necessarily. I own a Piaggio MP3 with two front wheels, and it is not a trike, it is a scooter. "Trike" is not recognized in some states for bikes that do not have two rear wheels.

In other words, it all depends on who is doing the defining...

How are we going to classify the spyder?

As mLewisLogic said, they always have 2 wheels. The word motorcycle is derived from the word bicycle, but since it's also having an engine it then became motorbicycle. Then shortened to what we know now as motorcycle or over here in blighty a motorbike.

Anything called a motorcycle has 2 wheels (sidecars aside... no pun intended).

If it has 3 wheels, it's called a trike.

I agree with Catatonic. Many jurisdictions define a motorcycle as "any motorized vehicle over 50cc that has less than four wheels". This is what defines a motorcycle in my state, for instance.