The terms skin diving, scuba diving, freediving, snorkelling, have an interesting past.
Before skin diving there was hard hat diving (as mentioned already by a previous poster).
Down in San Diego and LA, CA., sometime starting back in the 1930's there was a group of guys that later became known as the Bottom Scratchers that practiced breath hold diving with only a pair of swimming trunks on. There were no wetsuits, masks or fins yet. These guys would freeswim down to 30 -40 feet and grab abalone, spear fish and so forth. One of them started using a pair of homemade googles that made it easier to see but had no nose pocket so they couldn't equalize the mask and they would get racoon eyes and get resuling injuries. One of them had the bright idea to put in ear plugs that kept water out but had disastrous effects on ones hearing. After time these divers became known as skin divers because they dove in nothing but their bare skin. Eventually someone came up with fins and wetsuits also came along sometime in the 40's (?). But the term "skin divers" stuck even when wetsuits came along because it was an established term. Even when the Aqualung came along which as also called the breathing lung or just 'lung the overall activity was still called skin diving. Scuba was a term that was an acronim for Self Contained Underwater Breathing Apparatus, (I'm sure everybody knows that). In the 50's Healthways put out a double hose reg that had the word "SCUBA" with self contained breathing apparatus stamped on a plaque on the face of the can of the reg. I have one of these in working order BTW. I'm not sure, but it is possible that at this time scuba was used to describe tank diving. I've watched a few reruns of Sea Hunt where Bridges tells a diver to put his SCUBA on and go skin diving. I know the show was 50's corny with the acting as was full of rediculous scenarios, but the diving and terminology was real.
In the late 60's Skin Diver magazine almost was forced to change the name of the magazine to something other than "Skin Diver" because the term was considered vulgar and had sexual overtones by a the self proclaimed decency police (get your mind out of the gutter!)
Somewhere around this time the term was pretty much lost. Some people still used the term to describe breath hold diving and still do, but that would be a slight misnomer.
I don't know when the term freediving came along. I would have to reference one of my books and see if I can find it. I'm guessing sometime in the 70's or early 80's when tournament breath hold spearfishing became popular. I think there might have been some contempt between scuba divers and breath hold divers so the breath holders separated themselves by coining the word "FREE" to separate themselves as the "real divers" or purists.
So, in reality is is possible to do all these different activities in one operation if you think about it. We could SNORKEL out to the dive site in our SKIN (no wetsuit) we use SCUBA to do our dive. The only thing we can't do while were out there realistically is freedive.