What is atx formfactor?

If this is related to Personal Computers, it refers to the design specification for the size and shape of the motherboard, which is the heart of the device. An ATX "case," which is the enclosure for the computer is designed to incorporate a motherboard using the ATX form factor meaning it has holes positioned to accept screws that attach the motherboard to the case as well as external slots that allow add-on cards to be accessible out the back of the case, like video or audio cards, etc.