form factor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A specification for the physical dimensions and configuration of a device or component.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The ratio of the RMS value to the absolute mean of a sinusoidal wave (especially to that of an alternating current)
• noun physics Any of several functions that describe the unknown internal state of a particle
• noun physics The emissivity of a material
• noun crystallography A function that describes the scattering power of an atom as function of the scattering angle
• noun mechanics A factor describing the stress distribution of a body
• noun The geometry of an object, especially in engineering design; configuration.