What makes a band/artist a "rock" band/artist?

OK, the "Top 40" thread prompted me to ask this question. Just what makes a band (or artist) a rock band? My hubby & I argue this question fairly regularly. For example, he doesn't consider Styx (as he knows them) to be a rock band. (Please forgive me for marrying him!) I mean how can you not consider the band that gave us Rockin' the Paradise, Borrowed Time, Midnight Ride, Miss America, Renegade, etc. a "rock" band?!
Now I realize this is a very subjective question, based solely on each individual's definition of a "rock" band. Just curious what others think.

Now I realize this is a very subjective question, based solely on each individual's definition of a "rock" band. Just curious what others think.