It's an "intuitive" word with no technical meaning. People debate the relative "smoothness" of different hubs, some mf in this very thread claimed that loose balls are "smooth" but phils are "smooth" in a different way... but no one can actually say what it means. I'm going to go out on a limb and guess that no bfssfger who runs their mouth about this stupid **** has ever looked at a chart comparing friction coefficients (or whatever the actual technical terminology is, I don't understand this stuff either) for different hubs. Let alone what kind of difference that would make on the track or off. My guess is, it's certainly not something you could ever notice without careful use of a stopwatch under controlled conditions. I could barely tell when my hub was just about locked up from a cartridge bearing exploding... it felt like a mild headwind.