Someone who was evil or misguided, usually with super powers, but sometimes just incredibly smart, mad scientists. They often committed crimes. Not all supervillains were evil. Some believed that they were serving the greater good of mankind, but they took immoral methods to serve the greater good.
There were many alternate terms used aside from supervillain. One such term was super-criminal or just criminal.
In some cases, that term wasn't used; and neither was supervillain, due to it not being entirely accurate, but the term bad guy or evildoer or even outlaw might be used instead.