• synonyms


  1. (of a straight-sided plane figure) having two sides equal: an isosceles triangle; an isosceles trapezoid.
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Origin of isosceles

1545–55; < Late Latin < Greek isoskelḗs with equal legs, equivalent to iso- iso- + skél(os) leg + -ēs adj. suffix
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for isosceles

Historical Examples

  • In an isosceles triangle the angles opposite the equal sides are equal.

    The Teaching of Geometry

    David Eugene Smith

  • And equilaterals, and isosceles triangles, and all that, scoffed Neale.

  • So they crept up to the fire in the formation of an isosceles triangle.

    Denis Dent

    Ernest W. Hornung

  • Then ask them if they are equal-legged (isosceles) triangles.

  • That the angles at the base of an isosceles triangle are equal.

British Dictionary definitions for isosceles


  1. (of a triangle) having two sides of equal length
  2. (of a trapezium) having the two nonparallel sides of equal length
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Word Origin

C16: from Late Latin, from Greek isoskelēs, from iso- + skelos leg
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for isosceles


"having two equal sides," 1550s, from Late Latin isosceles, from Greek isoskeles "with equal sides," from isos "equal" (see iso-) + skelos "leg" (see scalene).

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Online Etymology Dictionary, © 2010 Douglas Harper

isosceles in Science


  1. Of or relating to a geometric figure having at least two sides of equal length.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.