R5RS: Legacy Scheme
The The Revised5 Report on the Algorithmic Language Scheme defines a dialect of Scheme. We use R5RS to refer to both the standard and the language defined by the standard.
See Dialects of Racket and Scheme for general information about different dialects of Scheme within Racket.
The default dialect of Lisp provided by racket and other Racket tools differs from R5RS in many ways, but Racket includes tools and libraries for running R5RS programs.