the annual what is a fly debate is 2 months early. I didn't expect to see this until March
Eventually after all the debate is over it can be defined in two different ways; an angler's choice of what is a fly, and what the regulations say is a fly. Neither of these is cut and dry. On one extreme view point all streamers are called a lure, because they do not imitate an insect. On the other extreme is the definition of if I can cast it with a fly rod its a fly.
My own opinion is to look at the appropriate regulations and follow them.