Form factor refers to the linear dimensions and configuration of a device as distinguished from other measures of size (for example Gigabytes; a measure of storage size):

• in computing, form factor is used to describe the size and format of PC motherboards (see AT, ATX, WTX, BTX), but also of hard drives, power supplies, cases, and add-in cards. The term can also be used refer to the shape of a housing or package or mechanical connection associated with a device or mechanism within the context of its interface with other devices or mechanisms, also in regards to a human interface.
• in engineering, the form factor is the proportion of radiation leaving one surface which is intercepted by another surface. This meaning of form factor is used in radiative heat transfer , illumination engineering , and computer graphics.
• in physics there are electric form factor and magnetic form factor which are Fourier transforms of charge and current distributions respectively.