XR is short for the umbrella term "eXtended Reality", though I suspect (without proof) the latter started as a backronym, with the X perhaps originally meaning

XR is short for the umbrella term "eXtended Reality", though I suspect (without proof) the latter started as a backronym, with the X perhaps originally meaning the traditional placeholder "whatever" for "Augmented", "Virtual", "Mixed" etc, which "Extended Reality" encompasses. Eg. https://en.wikipedia.org/wiki/Extended_reality https://en.wikipedia.org/wiki/Extended_reality