Automatic (or semi-automatic, they are essentially the same thing with slightly different mechanicals) means self acting, and with firearms that means 'self loading' for the most part.is a double-action pistol considered a semi-automatic weapon or is it in the same category as a revolver?
So, with a revolver you have to cock the hammer to rotate the cylinder bring the next piece of ammunition to bear. DA revolvers essentially just connect the trigger so that when it's pulled the hammer cocks and the cylinder rotates and it fires.
Now pistols, semi auto, DA or SA, are self acting mechanisms. They all have to have a round chambered initially but when you fire it, they load the next round and/or cock the firing mechanism so that all you have to do is pull the trigger. [In a DA pistol, the trigger pull cocks the firing mechanism and fires all in one go. Usually why there are two distinct parts of the trigger pull]
The major difference is that one has to be manually operated and one is self acting. Hence, revolvers are not semi-auto.