The Collaborative International Dictionary of English v.0.48:

minicomputer \minicomputer\ n. (Computers)
   a mid-sized digital computer; at any given point in the
   development of computer technology, a minicomputer will be
   faster and have greater capacity than a microcomputer, but
   will be slower and have less capacity than a mainframe
   computer.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

minicomputer
    n 1: a digital computer of medium size

The Free On-line Dictionary of Computing (30 December 2018):

minicomputer

    A computer built between about 1963 and 1987,
   smaller and less powerful than a mainframe, typically about
   the size and shape of a wardrobe, mounted in a single tall
   rack.

   Minicomputers were characterised by short word lengths of 8
   to 32 bits, limited hardware and software facilities and
   small physical size.  Their low cost made them suitable for a
   wide variety of applications such as industrial control, where
   a small, dedicated computer which is permanently assigned to
   one application, is needed. In recent years, improvements in
   device technology have resulted in minicomputers which are
   comparable in performance to large second generation
   computers and greatly exceed the performance of first
   generation computers.

   The processor was typically built using low integration logic
   integrated circuits - TTL or maybe ECL, thus
   distinguishing it from a microcomputer which is built around
   a microprocessor - a processor on a single (or maybe a few)
   ICs.

   DEC's PDP-1 was the first minicomputer and their PDP-11
   was the most successful, closely followed (in both time and
   success) by the VAX (which DEC called a "super
   minicomputer").

   Another early minicomputer was the LINC developed at MIT in
   1963.

   Other minicomputers were the AS/400, the PRIME series, the
   AP-3, Olivetti's Audit 7 and the Interdata 8/32.

   [Others?]

   (2004-05-12)