The Collaborative International Dictionary of English v.0.48:

XP \XP\ [Belongs here in appearance only.]
   The first two letters of the Greek word XRISTOS, Christ; --
   an abbreviation used with the letters separate or, oftener,
   in a monogram, often inclosed in a circle, as a symbol or
   emblem of Christ. It use as an emblem was introduced by
   Constantine the Great, whence it is known as the

   Constantinian symbol, or

   monogram. See Labarum.
      [Webster 1913 Suppl.]

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

XP
       [Windows] eXPerience (MS, Windows, OS)


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

XP
       eXtreme Programming


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

Windows XP
XP

    The version of the Microsoft Windows
   operating system that, when it was released on 2001-10-25,
   finally merged the Windows 95 - Windows ME strain with the
   Windows NT - Windows 2000 one.  XP comes in two main
   versions: Windows XP Professional Edition and a simplified
   subset for home users, Windows XP Home Edition.

   Windows XP home page (http://microsoft.com/windowsxp/).

   (2009-08-12)