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

glass tty
 /glas T?T?Y/, /glas ti'tee/, n.

    [obs.] A terminal that has a display screen but which, because of hardware
    or software limitations, behaves like a teletype or some other printing
    terminal, thereby combining the disadvantages of both: like a printing
    terminal, it can't do fancy display hacks, and like a display terminal, it
    doesn't produce hard copy. An example is the early ?dumb? version of
    Lear-Siegler ADM 3 (without cursor control). See tube, tty; compare 
    dumb terminal. See TV Typewriters (Appendix A) for an interesting true
    story about a glass tty.


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

glass tty

   /glas T-T-Y/ or /glas ti'tee/ A terminal that has a display
   screen but which, because of hardware or software limitations,
   behaves like a teletype or some other printing terminal,
   thereby combining the disadvantages of both: like a printing
   terminal, it can't do fancy display hacks, and like a display
   terminal, it doesn't produce hard copy.  An example is the
   early "dumb" version of Lear-Siegler ADM 3 (without cursor
   control).  See tube, tty; compare dumb terminal, smart
   terminal.  See "TV Typewriters" for an interesting true
   story about a glass tty.