Trucking: semi trucks, semi trucks, two wheels

Question
why are they called "semi" trucks. I'm dying here, I can't find the answer anywhere online!  

Answer
I have head other theories but I think when they were first invented most trailers or carts had 4 wheels and were pulled by horses.  When they hooked the front of the trailer to the vehicle and it only had two wheels it became known as a "semi-trailer"  Here is a short article:
http://www.selectyourtruckdeal.com/freightliner.htm
Jim