Wikipedia
FlexATX
FlexATX is a motherboard form factor derived from ATX. The specification was released in 1999 by Intel as an addendum to the microATX specification. It uses a subset of the motherboard mounting holes required for microATX and the same I/O plate system as ATX and microATX.
FlexATX specifies that a motherboard be no larger than , and can have no more than three expansion slots.
The term is used also for the form factor of a PSU that is smaller than a standard ATX PSU and is used in small cases that host a FlexATX or Mini ITX motherboard or in thin rackmount servers such as 1U racks.