V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

XPG
       X/open Portability Guide (X/Open, X-Windows)


The Free On-line Dictionary of Computing (30 December 2018):

X/open Portability Guide
XPG

   (XPG) A document which defines the interfaces of the X/Open
   Common Applications Environment.

   Version: XPG3.

   (1994-12-01)