slide rule

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A device consisting of two logarithmically scaled rules mounted to slide along each other so that multiplication, division, and other more complex computations are reduced to the mechanical equivalent of addition or subtraction.

from The Century Dictionary.

• noun A sliding rule. See slide.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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 This device has been largely superseded by the electronic calculator, which has a greater precision than the slide rule.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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An analog calculator consisting of three interlocking strips marked with logarithmic scales, such that multiplication, division etc. can be performed by the equivalent of addition and subtraction.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun analog computer consisting of a handheld instrument used for rapid calculations; have been replaced by pocket calculators