The Collaborative International Dictionary of English v.0.48:

Slide rule \Slide" rule`\, n.
   a thin, flat calculating device consisting of a fixed outer
   piece and a movable middle piece. Both pieces are graduated
   in such a way (as, by a logarithmic scale) that
   multiplication, division, and other mathematical functions of
   an input variable may be rapidly determined by movement of
   the middle pieces to a location on one scale corresponding to
   the input value, and reading off the result on another scale.
   A movable window with a hairline assists in alignment of the
   scales. This device has been largely superseded by the
   electronic calculator, which has a greater precision than the
   slide rule. Also called colloquially slipstick.
   [PJC]

WordNet (r) 3.0 (2006):

slipstick
    n 1: analog computer consisting of a handheld instrument used
         for rapid calculations; have been replaced by pocket
         calculators [syn: slide rule, slipstick]