MzScheme is an implementation of the Scheme programming language for Windows, MacOS, Unix. With the renaming of PltScheme to Racket, MzScheme (the program) has been renamed just Racket. A legacy language called MzScheme still exists for backwards compatibility with old MzScheme programs.
See RacketLanguage.
http://docs.racket-lang.org/mzscheme/index.html