The Collaborative International Dictionary of English v.0.48:

mainframe \main"frame`\ n. (Computers)
   1. A large digital computer serving 100-400 users and
      occupying a special air-conditioned room. At any given
      point in development of computer technology, the mainframe
      will be faster, have large main memeory, and be more
      capable than a minicomputer, which will in turn be
      faster and more capable than a personal computer. The
      typical personal computer in 1999 is faster than a
      mainframe was in 1970.

   Syn: mainframe computer.
        [WordNet 1.5 +PJC]

   2. The board holding the CPU and the memory forming the
      central part of a computer to which the peripherals are
      attached.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

mainframe
    n 1: a large digital computer serving 100-400 users and
         occupying a special air-conditioned room [syn: mainframe,
         mainframe computer]
    2: (computer science) the part of a computer (a microprocessor
       chip) that does most of the data processing; "the CPU and the
       memory form the central part of a computer to which the
       peripherals are attached" [syn: central processing unit,
       CPU, C.P.U., central processor, processor,
       mainframe]

The Jargon File (version 4.4.7, 29 Dec 2003):

mainframe
 n.

    Term originally referring to the cabinet containing the central processor
    unit or ?main frame? of a room-filling Stone Age batch machine. After the
    emergence of smaller minicomputer designs in the early 1970s, the
    traditional big iron machines were described as ?mainframe computers? and
    eventually just as mainframes. The term carries the connotation of a
    machine designed for batch rather than interactive use, though possibly
    with an interactive timesharing operating system retrofitted onto it; it is
    especially used of machines built by IBM, Unisys, and the other great 
    dinosaurs surviving from computing's Stone Age.

    It has been common wisdom among hackers since the late 1980s that the
    mainframe architectural tradition is essentially dead (outside of the tiny
    market for number-crunching supercomputers having been swamped by the
    recent huge advances in IC technology and low-cost personal computing. The
    wave of failures, takeovers, and mergers among traditional mainframe makers
    in the early 1990s bore this out. The biggest mainframer of all, IBM, was
    compelled to re-invent itself as a huge systems-consulting house. (See 
    dinosaurs mating and killer micro).

    However, in yet another instance of the cycle of reincarnation, the port
    of Linux to the IBM S/390 architecture in 1999 ? assisted by IBM ? produced
    a resurgence of interest in mainframe computing as a way of providing huge
    quantities of easily maintainable, reliable virtual Linux servers, saving
    IBM's mainframe division from almost certain extinction.


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

mainframe

    A term originally referring to the cabinet
   containing the central processor unit or "main frame" of a
   room-filling Stone Age batch machine.  After the emergence
   of smaller "minicomputer" designs in the early 1970s, the
   traditional big iron machines were described as "mainframe
   computers" and eventually just as mainframes.  The term
   carries the connotation of a machine designed for batch rather
   than interactive use, though possibly with an interactive
   time-sharing operating system retrofitted onto it; it is
   especially used of machines built by IBM, Unisys and the
   other great dinosaurs surviving from computing's Stone
   Age.

   It has been common wisdom among hackers since the late 1980s
   that the mainframe architectural tradition is essentially dead
   (outside of the tiny market for number crunching
   supercomputers (see Cray)), having been swamped by the
   recent huge advances in integrated circuit technology and
   low-cost personal computing.  As of 1993, corporate America is
   just beginning to figure this out - the wave of failures,
   takeovers, and mergers among traditional mainframe makers have
   certainly provided sufficient omens (see dinosaurs mating).

   Supporters claim that mainframes still house 90% of the data
   major businesses rely on for mission-critical applications,
   attributing this to their superior performance, reliability,
   scalability, and security compared to microprocessors.

   [Jargon File]

   (1996-07-22)