Merriam Webster defines drone as:
1 : the male of a bee (as the honeybee) that has no sting and gathers no honey
2 : one that lives on the labors of others : PARASITE
3 : an unmanned aircraft or ship guided by remote control
In the preceding list, definition #3 fits the bill for a wide range of UAV's, including RC quadcopters.
Furthermore, American Heritage Dictionary define drone as--in particular, see #4:
1. A male bee, especially a honeybee, that is characteristically stingless, performs no work, and produces no honey. Its only function is to mate with the queen bee.
2. An idle person who lives off others; a loafer.
3. A person who does tedious or menial work; a drudge: "undervalued drones who labored in obscurity" (Caroline Bates).
4. A pilotless aircraft operated by remote control.