When's a Fly Not Really a Fly?

I remember bumping into legendary fly developer Rene Harrop in the A-Bar, near Idaho's Henry's Fork, one evening, about 10 years ago. The fishing had been typically tough in the "Railroad Ranch" section of the river that day, but I had managed to catch one particularly nice 'bow on a giant, gaudy, foam Chernobyl ant. I showed the master the bug, and he snickered.

"Indian jewelry," he said.

Which still has me thinking ... when is a fly not really a fly? I've seen those "spoon flies" that folks throw at redfish in the Gulf, for example. C'mon, that's a lure you happen to throw with a fly rod, isn't it? Gummy minnows? Soft plastics for the surf. But is a foam hopper a faux fly or a faux pas?

Does a fly, by definition, have to be constructed almost wholly from materials that were once part of living animals (fur and feathers)? Or is a fly, by definition, any "bait" that isn't, well, live bait?

What say you, Fly Talkers?

Deeter