What is Class R&B?

I know how I define Classic R&B and where to find an accurate definition for Classic Rock (i.e.: Wikipedia) but I don't know where to find an accurate definition for Classic R&B. For me, Classic R&B begins with Motown and ends with the demise of Sugar Hill Records. For me, it includes funk (i.e.: Funkadelic) and old school rap (i.e.: Grandmaster Flash & The Furious Five) but predates gangster rap (i.e.: NWA). I don't know if this is an accurate description but, then again, I don't know where to find one.