It turns out to be potentially incorrect to describe these things - in distinction from rotary-wing devices - as 'generating lift from forward motion', because helicopters also generate increased lift due to forward motion; if you can taxi one down a runway, you can take off with a heavier load than if you have to take off from a fixed location. It is not due to the air meeting the blades faster, because what you gain on the forward-moving blades you lose on the backwards-moving blades. Rather, it is due to more complex causes having to do with the pattern of the airflow around the vehicle. See this page and this page for more.
The best definition is the one implied in the name: the lifting surfaces do not move relative to the rest of the structure. J. Noel Chiappa 12:52, 21 May 2008 (CDT)