Form Factor
The physical shape and proportions of an object's outer volume, especially its housing.
In practice
Form factor is the outer size, shape and proportions of a product's body, the envelope that its internal parts must fit into. It is a key early decision in industrial design because it shapes everything that follows: a speaker shaped as a tall cylinder places drivers and buttons very differently from a flat puck. In electronics, the term also names standard size categories, such as laptop versus tablet.
Often confused with Form
Form factor is a product's overall outer envelope and proportions; form is the fuller perceived shape, including details and transitions between surfaces.
Try this
Measure three handheld devices you own and sketch each as a plain block, comparing proportions only.