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

Short Message Service

    (SMS) A message service offered by the GSM
   digital mobile telephone system.

   Using SMS, a short alphanumeric message (160 alphanumeric
   characters) can be sent to a mobile phone to be displayed
   there, much like in an alphanumeric pager system.  The
   message is buffered by the GSM network until the phone becomes
   active.

   (1996-02-18)