In the above diagram, $Q$ is the apex.
Also known as
- Definition:Apex of Tetrahedron: all the faces of a tetrahedron are triangular, so this definition cannot apply as it stands.
The plural of apex is apices, which is pronounced ay-pi-seez.
The form apexes can often be seen, but this is technically incorrect.
Hence the colloquial phrase base over apex as the description of a particularly flamboyant physical tumble.