In the above diagram, if $AC$ is taken to be the base of $\triangle ABC$, then $B$ is the apex.
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.
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): Entry: base (of a triangle)