According to Dictionary.com, ATX stands for Advanced Technology eXtended. ATX, first described by Intel in 1995, is a motherboard form factor. Though the years, the standard has been updated and expanded.
The original ATX form factor is still in use today, but variations of it have become popular as well. By standardizing on specific power levels, connection types and sizes, manufacturers can design hardware guaranteed to work with other components. These specifications help foster competition and prevent users from having to purchase new computers when they want to upgrade a particular component. While there are other form factor families, the ATX line dominates consumer-grade computing.