capitalize-word is an interactive built-in function in `C source code'.

It is bound to M-c.

(capitalize-word ARG)

Capitalize from point to the end of word, moving over.
With numerical argument ARG, capitalize the next ARG-1 words as well.
This gives the word(s) a first character in upper case
and the rest lower case.

If point is in the middle of a word, the part of that word before point
is ignored when moving forward.

With negative argument, capitalize previous words but do not move.