Definition:Isosceles Triangle/Also defined as

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Isosceles Triangle: Also defined as

Some sources require that in an isosceles triangle, the third side has to be specifically of a different length than the two defined as being equal.

Hence, under such a definition, an equilateral triangle would not be classified as isosceles.


Euclid's Definition

In the words of Euclid:

Of trilateral figures, an equilateral triangle is that which has its three sides equal, an isosceles triangle that which has two of its sides alone equal, and a scalene triangle that which has its three sides unequal.

(The Elements: Book $\text{I}$: Definition $20$)


Sources