Minicomputer is a now largely obsolete term for a class of computers. The term evolved in the 1960's to describe "small" server-class computers, usually taking up one or a few cabinets, compared with mainframes that would usually fill a room. They were far less expensive than mainframes.
As microcomputers developed in the 1970s and 80s, minicomputers filled the mid-range area between micros and mainframes. Microcomputers were single-user, relatively simple machines running simple program-launcher operating systems like CP/M or MS-DOS, while minis were much more powerful systems that ran full multi-user, multitasking operating systems like VMS and Unix.
As of 2001, the term minicomputer is no longer used for the mid-range computer systems, and most are now referred to simply as servers.
This paragraph really needs work: Where mainframes had 32 or 64-bit words, minicomputers often had 12 or 16-bit words. Where mainframes might have originally had 500K of RAM, minicomputers might have 4K. Where a mainframe might have 750 instructions, a minincomputer might have had 8.
Today's personal computer are descendants of the microcomputers, but architecturaly their CPUs and operating systems have evolved largely by integrating features from minicomputers.
The class of minicomputers includes:
See also History of computing