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The vertex of a pyramid which is the common vertex of its triangular faces is called the apex of the pyramid.

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

Also known as

The apex of a pyramid is seen in some sources as vertex.

Also see

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, whose plural is vertices.

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