isosceles triangle

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isosceles triangle (plural isosceles triangles)

  1. (geometry) A triangle having two sides equal.

Usage notes[edit]

In some mathematical usage, an isosceles triangle has at least two sides equal, and any triangle with three sides equal also has two sides equal, so, in this view, all equilateral triangles are also isosceles. In original usage, an isosceles triangle is understood to have the third side unequal to the other two, that is, an isosceles triangle has exactly two equal sides.