This designation came from England more than 65 years ago and orginally was meant to convey the idea that the scale was Half of O.  The proportion is 1:87, or 3.6 millimeters to the foot.  Track measures .66 inch (16.6 mm) between the running rails.

 

HO has been the overwhelming choice among scale model railroaders for a long time.  The story of HO toy trains is different.  Maybe because of their size they could not withstand abuse from children as well as could larger trains.  In spite of excellent marketing and competitive pricing, HO toy train sets never really caught on.  Some are still available, but they represent a small percentage of toy train sales.