It is incomplete, at best. Mountain wave is a condition of very strong updrafts and downdrafts that continue in a steady state leeward of a peak or ridge, caused by certain conditions but almost always involving a strong wind at the peaks and high-pressure air. It can extend many miles, or hundreds of miles, downrange. You can actually pick it out on satellite pretty well, if you know what to look for. Glider pilots love it... power pilots, not so much.
You can think of ripples in a fast-moving creek caused by a submerged log that you can't see, and you'll get the idea.
A downdraft is just a descending air mass -- its cause is not part of its definition. Conditions experienced in the mountains can very much be classified into updrafts and downdrafts, as well as sideways drafts and, where the wind is blowing hard enough I swear to goodness "inside-out" drafts. The wind does amazing things. Picture again turbulent flow of a flooded creek, and you get the idea. (Is there anything that water analogies CAN'T solve? The world wonders)
~Fox