Definition I found online:

?Jigging is the practice of fishing with a jig, a type of fishing lure. A jig consists of a lead sinker with a hook molded into it and usually covered by a soft body to attract fish. Jigs are intended to create a jerky, vertical motion, as opposed to spinnerbaits which move through the water horizontally.?
I'm sure that definition would vary somewhat depending on where you find it. But by that definition, the flies I tie on jig hooks are not jigs because I don't fish them in a vertical motion. I would also guess the first tyer who used synthetic materials was scoffed at, too. I don't see that what type of hook you tie your imitation on makes it a "fly" or not.

Joe