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

control-Q
 vi.

    ?Resume.? From the ASCII DC1 or XON character (the pronunciation /X-on/
    is therefore also used), used to undo a previous control-S.


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)