Originally a scripting language developed at IBM. Rexx was the successor to the script languages EXEC and EXEC 2. Originally called REX, the first specification was dated 1979-03-29 and was written prior to any implementation. It was first implemented by Mike Cowlishaw at IBM.
In 1996 ANSI published a standard for REXX: ANSI X3.274-1996 "Information Technology – Programming Language REXX"
Cowlishaw seems to prefer Rexx, whereas IBM sales, ANSI, and the majority of the web uses REXX.