A motherboard form factor released by Intel in 1997. It draws heavily on their ATX form factor, sharing the same 20-pin power connector, I/O shield, power control, socket placement, and slot placement, although it only has a maximum of 4 slots (usually 1 AGP and 3 PCI).
It can use either a standard ATX power supply or a SFX power supply to save space. Its dimensions are 244mm fixed height (along slots and I/O shield) x maximum 244mm width.
note: this form factor is now officially superceded by FlexATX, but is still in widespread usage and by no means deprecated in reality.
The full specification is available at http://www.formfactors.org/developer/specs/microatx/microatxspecs.htm