Although we think of rock and roll music as an invention of the 50's it is interesting to note that in an issue of Billboard from April 21, 1945 we find the following:
Something was being referred to as "rock and roll music" and it was apparently known to the reader what this described. This warrants further investigation, though it's the last sentence I love: : "the phono fans seeking out the jump incentive in the spin, will shower plenty favor on 'Caldonia'"
That's us. Seeking out the jump incentive in the spin.