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.