R 'n' b does indeed stand for Rhythm 'n' Blues, but its name seems to have expanded as a musical term, as time has gone on. Inititally coined as a term to define the new 'jump blues' music of the 1950's, including artists such as Ray Cahrles and James Brown, the label expanded to include the 'beat' sounds of the 1960's, in much the same way as 'Heavy Metal' began defining bands like Black Sabbath, and now includes a vast array of bands with a sound based in 'Metal', but not exclusive to it.
Similarly, R 'n' B as a musical type has expanded from its original fairly narrow definition, to include most of today's soul and dance-based music, usually, although not exclusively defined by rhythms and textures based in black music. It all seems to evolve from a need to place music in carefully defined boxes, which is self-defeating, because, as you point out Waterwolf, no sooner has someone somewhere put a label on a musical genre, than it expands and disolves its borders to include all sorts of other similar styles, and its original meaning becomes lost, leading to another set of labels, perpetuating the labeling system.
So Mick Jagger, Marvin Gaye, and Destiny's Child all make the same sort of music do they? Case rested.