There were similar terms used (pistala and pistole) in Europe in the mid 1500s to describe a gun fired with the hand.
That was the standard term until the mid 1800s when the pepperbox and, around the same time, the first Colt revolvers were introduced. Both were often advertised as revolving pistols.
This advertisement for the Colt Single Action Army, from around 1873-1875, illustrates that:
http://www.icollector.com/Original-Colt-broadside-paper-advertisement-for-the-Colt-SAA-revolver-and_i23720784Colt also referred to them as repeating pistols in ads for the earlier cap and ball revolvers.
Not exactly sure when they started to be called revolvers, but Smith & Wesson was using the term revolver in the 1880s.
Around the introduction of semi-automatics, the terms automatic and automatic pistol were in common use.
It seems that very slowly, and likely driven by such advertisements, revolver became the common term for the wheel gun and pistol sort of morphed over to mean a semi-auto repeater.
No one ever really decided it, it just happened.