Form
The perceived shape and structure of an object, independent of its color or surface texture.
In practice
Form here means an object's perceived shape and structure, considered apart from its color or texture. Designers often study form in plain gray or white to judge it honestly. A water bottle's form is its profile, the way its shoulder curves into the neck and how its body sits in the hand; painting it bright colors changes how it feels, but not its form. This sense is unrelated to a web form for entering data.
Often confused with Form Factor
Form is the full perceived shape and structure of any object; form factor is specifically the outer size and proportions of a product's housing.
Try this
Wrap a familiar object in white paper or foil and describe its shape without mentioning color or material.