5th gear - overdrive?

I've heard a few guys on here refer to 5th gear as "overdrive". Though I think I can guess what they mean, I'd like to be sure - so what, exactly, do you mean by that? To me, when the bike is near 50 mph or more, it "wants" to be shifted into 5th gear. Some have said they drive in 4th gear at even faster speeds and do it for long periods of time; this is okay for the engine?

PS - I think you guys who ride these little bikes at 75 mph or faster are nuts - not dissing you, even admire your balls, but Jesus, the bike does not feel stable to me at that speed, especially if there is wind or other mitigating factors (heavy traffic, wet roads, etc.).

Thanks,

Keith