In the above diagram, $BC$ is the base.
In the above diagram, $\angle ABC$ and $\angle ACB$ are the base angles.
In the above diagram, $A$ is the apex.
In the above diagram, $AB$ and $AC$ are the legs.
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.
- Results about isosceles triangles can be found here.
The word isosceles comes from the Greek: $\iota \sigma \omicron \sigma \kappa \epsilon \lambda \epsilon \varsigma$, that is: from iso meaning equal, and skelos meaning leg.
Thus an isosceles triangle is literally an equal-leg triangle.
It is pronounced eye-sos-ell-eez, that is, with the emphasis on the second syllable. Note that the c is silent.
The word skeleton comes from the same linguistic root.
- 1989: Ephraim J. Borowski and Jonathan M. Borwein: Dictionary of Mathematics ... (previous) ... (next): triangle (Euclidean geometry)
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): triangle
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): triangle
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): isosceles triangle