Real Dictionary

shim in American English
a thin, usually wedge-shaped piece of wood, metal, or stone used for filling space, leveling, etc., as in masonry
to fit with a shim or shims
noun: a thin slip or wedge of metal, wood, etc., for driving into crevices, as between machine parts to compensate for wear, or beneath bedplates, large stones, etc., to level them
transitive verb: to fill out or bring to a level by inserting a shim or shims

shim in British English
noun: a thin packing strip or washer often used with a number of similar washers or strips to adjust a clearance for gears, etc
verb: to modify (a load, clearance, or magnetic field) by the use of shims

shim in Mechanical Engineering
noun: A shim is a small object or piece of material used, for example, to create a little extra height or a little extra space.