Other than a spinning rod and reel is not used and the casting technique........ no difference
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II
I don't have any knowledge of the technique, so I assumed that if no fly line is used and the weight of the lure pulls the line off some kind of reel (or off the ground, whatever) that it is similar to ultralight spin fishing (though I agree it is technically not "spinning" if a spinning reel is not used). An argument can be made that the rod doesn't make a difference for purposes of definition; after all, there are combo fly-spin rods. In fact, I am going to put a fly reel and line on one of my spinning rods and see how it works. I will let you know.
Anyway, as Voltaire supposedly said (probably apocryphal), "If you would converse with me, you must first define your terms." So how do we define fly fishing? I would suggest two criteria:
1. The weight of the line carries the "lure," rather than a heavy lure pulling out light line.
2. The "lure" is a fly.
Of course, then we have to define "fly." You can use your fly rod and line to cast a hook with a worm on it, but I don't think most of us would consider that fly fishing (though I could be wrong). I have seen these tiny spoons made for use with a fly rod; is that fly fishing? And I think some people consider a Clouser to be really a jig. So what is a fly?