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

XON
       eXchange ON (MODEM)


The Jargon File (version 4.4.7, 29 Dec 2003):

XON
 /X?on/, n.

    Syn. control-Q.


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

control-Q
DC1
XON

    (Or XON, DC1, Device Control 1) The character
   with ASCII code 17, used in software handshaking to resume
   output after a previous control-S.

   [Jargon File]

   (1996-06-28)