An Acronym for Extended Graphics Array.
The graphics hardware standard intended by IBM to supersede VGA. It adds resolutions up to 1024x768 pixels in 256 colours (4:3 aspect ratio). However, it was built for the MCA bus which never caught on, and required special drivers to use as more than a plain VGA card, so XGA never became successful. The market was won over by a slew of incompatible SVGA designs instead.
The acronym was later recycled by MarkeTroids to mean a maximum resolution support of 1024x768 pixels in LCD displays and computer video projectors. Technically, they have nothing to do with XGA. This was then bastardized further as maximum resolutions grew: