Acronym for 'crystal set'; Simplistic radio receiver with basic tuner/detector circuitry. Signal power of radio station effects 'detection' of radio frequency signal and 'excites' audio signal in earphone circuitry:
Academic & Science » Electronics
XT - XT* - XT3 - XTA - XTAL - XTB - XTC - XTCU - XTD - XTE
"XTAL SET." Abbreviations.com. STANDS4 LLC, 2018. Web. 18 Feb. 2018. <https://www.abbreviations.com/XTAL%20SET>.