When in doubt look it up in the Oxford English Dictionary...it is rather explicit and exact in it's true meanings of words. Might be more useful then researching what's commonly being used erroneously.
Looks like they are interchangeably used, yet they are NOT the same word. Skein referring to the rather loose coils of yarn specifically. And Hank referrs to many materials coiled and gathered.
So a Skein is a hank (of yarn/thread) yet a hank isn't nescesarily a Skein. Isn't the English language deliciously deceptive and confusing?