Rock n roll is a definable thing. It may be a broad catagory (Elton John vs Metal, for example), but it is NOT all inclusive.
Industrial/house/dubstep: NOT rock n roll
Slow jam: NOT rock n roll.
Hip Hop: Is SOMETIMES rock n roll.
Taylor Swift: NOT rock n roll.
Johnny Cash: SOMETIMES rock n roll.
Pat Boone: IS rock n roll when he's off his meds, is NOT rock n roll when he's on them.
Country Music in general: CAN be rock n roll, but usually isn't.
Jazz: NOT rock n roll.
Blues: NOT rock n roll, but is in fact rock n roll's daddy.
Emotard shit: NOT NOT NOT rock n roll
Punk (real punk): Was USUALLY rock n roll.