Dictionary.com
definitions

rhythm-and-blues

[rith -uh m-uh n-blooz]
noun
  1. a folk-based but urbanized form of black popular music that is marked by strong, repetitious rhythms and simple melodies and was developed, in a commercialized form, into rock-'n'-roll.

Origin of rhythm-and-blues

An Americanism dating back to 1945–50
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for rhythm and blues

rhythm and blues

noun
  1. (functioning as singular) any of various kinds of popular music derived from or influenced by the bluesAbbreviation: R & B
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012