Constantly shifting thats how they are defined at least. Mini computer usually refers to the size of the system physically, some form factors that fit the Mini category are mini ITX, ITX, and mini ATX. I'm not certain if BTX has a "mini" variant or not but it never really picked up. Mainframes are sort of a thing of the past and modern mainframes are usually synonymous with supercomputers. However, several supercomputers are actually composed of "server farms," or dozens if not hundreds of individual computers which are networked together and process data as if they were all just one system. This type of super computer is known as a cluster, or parallel processing farm, or server farm and many technologies exist to make this happen. Google and Pixar are to big names that I know of which are using these large cluster computer farms. If you are interested in more do a google search for beowulf clustering.
