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

X.25
ISO 8208

    An ITU-T standard protocol suite
   for the DTE-DCE interface in a packet-switched network,
   approved by ISO.  X.25 defines standard physical layer,
   data link layer and network layers (layers 1 through 3).
   It was developed to describe how data passes into and out of
   public data communications networks.  X.25 networks are in use
   throughout the world.

   Document: ISO 8208.

   Several other ITU-T recommendations are relevant to packet
   switching: X.3, X.28, X.29, X.75.

   (1996-08-10)