V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

ATX
       Advanced Technology eXtended [form factor]


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

ATX

    An open PC motherboard
   specification by Intel.

   ATX is a development of the Baby AT specification with the
   motherboard rotated 90 degrees in the chassis.  The CPU and
   SIMM sockets have been relocated away from the expansion
   card slots meaning that all the slots support full-length
   cards.  More I/O functions are integrated on the
   motherboard.  As the longer edge of the board is now at the
   back of the chassis, there is more space for connectors; also,
   the I/O opening on the back panel of the chassis has been
   defined as double the previous height, allowing vendors to add
   extra on-board I/O functions over and above the standard.

   Most Pentium Pro boards use this form factor.

   As well as the motherboard size, layout, and placement, the
   ATX specification also includes requirements for power supply
   and fan specification and location.

   The full size ATX board measures 305mm wide by 244mm deep.
   There is also a Mini-ATX form factor, 284mm by 208mm.

   Home
   (http://developer.intel.com/design/motherbd/atx.htm).

   (2001-07-16)