A motherboard form factor that is smaller than the standard ATX size of 300mm x 240mm. The Mini-ITX is 170mm x 170mm (slightly smaller than the micro-ATX) and reduces the size of standard desktops and computer towers. Mini-ITX was developed by VIA Technologies.
See What You Need to Know About Motherboards in the Quick Reference section of Webopedia.
See also From AT to BTX: Motherboard Form Factors.