RE: Highwind / Lightwind definitions
In my opinion, both James and Del Carpenter raise interesting concerns. From my perspective, I see things similarly as James, in that I view higher winds starting in the 15-18 knot average range. Of course, some might view high winds in the 25+ knot range, and I really can't disagree. Yet much is grounded in the locale and what's considered normal average conditions.
Overall, I like to consider the leverage angle where somewhat lighter, but consistent average winds in the 15-18 knot range allows one to take advantage of smaller sails. Wave sailors are usually so inclined. This is where Del's point can mix things up a bit. In talking about light or high winds, a well understood guide or perspective makes real good sense, because most everybody can interpret and appreciate average ranges expressed in knots. Much more is communicated this way rather than referring to simply light or high wind conditions.
Lastly, Expander's last point of "mid range wind" is well worth note too, since the real world is always affecting our game. From my experience, there is usually a certain degree of maturation and change to the wind during the day. This includes wind strength and direction, but I believe that there in some real value in weighting the average mid range in the picture. What is "mid range"? That's debatable of course, but using knot averages, it could be easily defined.