The Collaborative International Dictionary of English v.0.48:

Shim \Shim\, n.
   1. A kind of shallow plow used in tillage to break the
      ground, and clear it of weeds.
      [1913 Webster]

   2. (Mach.) A thin piece of metal placed between two parts to
      make a fit.
      [1913 Webster]

WordNet (r) 3.0 (2006):

shim
    n 1: a thin wedge of material (wood or metal or stone) for
         driving into crevices

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

shim
 n.

    1. A small piece of data inserted in order to achieve a desired memory
    alignment or other addressing property. For example, the PDP-11 Unix
    linker, in split I&D (instructions and data) mode, inserts a two-byte shim
    at location 0 in data space so that no data object will have an address of
    0 (and be confused with the C null pointer). See also loose bytes.

    2. A type of small transparent image inserted into HTML documents by
    certain WYSIWYG HTML editors, used to set the spacing of elements meant to
    have a fixed positioning within a TABLE or DIVision. Hackers who work on
    the HTML code of such pages afterwards invariably curse these for their
    crocky dependence on the particular spacing of original image file, the
    editor that generated them, and the version of the browser used to view
    them. Worse, they are a poorly designed kludge which the advent of
    Cascading Style Sheets makes wholly unnecessary; Any fool can plainly see
    that use of borders, layers and positioned elements is the Right Thing (or
    would be if adequate support for CSS were more common).


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

shim

    A small piece of data inserted in
   order to achieve a desired memory alignment or other
   addressing property.

   For example, the PDP-11 Unix linker, in split I&D
   (instructions and data) mode, inserts a two-byte shim at
   location 0 in data space so that no data object will have an
   address of 0 (and be confused with the C null pointer).

   See also loose bytes.

   [Jargon File]

   (1994-12-21)