Rik's answer is sound (pun intended, as the whole thing relates back to music).  In physics, anything in sinusoidal oscillation is said to be in harmonic motion.  So, yes, the first harmonic is the fundamental frequency.

 

Now, if you want to talk about a REAL mystery (or so it seems to those of us outside the motherland, anyway) consider that first floor and first storey in a building are synonymous here, but are two different levels on that side of the pond.  That's even worse than overtones not being quite synonymous with harmonics.

 

:-)