Source : Free On-Line Dictionary of Computing

X.25
     
         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)