Definition:Triangle (Geometry)/Isosceles/Apex

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The vertex opposite the base of an isosceles triangle is called the apex of the triangle.

In the above diagram, $A$ is the apex.

Linguistic Note

The plural of apex is apices, which is pronounced ay-pi-seez.

The form apexes can often be seen, but this is technically incorrect.

Compare vertex.

Hence the colloquial phrase base over apex as the description of a particularly flamboyant physical tumble.