A programming language used in commercial data processing for extracting information from files. The input to an RPG consists of a description of the file structure, a specification of the information required, and of its layout on the page. From this information, the RPG constructs a program to read the file, extract the desired information, and format it in the required manner. The current version is RPG IV, also called ILE RPG.
http://www-01.ibm.com/support/knowledgecenter/api/content/nl/en-us/ssw_ibm_i_61/rzasc/sc092507.pdf The ILE RPG Programmer’s Guide