Friction is the resistance offered at the contact between two surfaces when one tends to move or moves relative to the other. It is a surface/contact phenomenon and is conventionally classified according to the nature of that relative motion.

The genuine categories of friction include:

• Static friction — the resistance between two surfaces that are at rest relative to one another; it self-adjusts up to a limiting value before motion begins.
• Kinetic (dynamic/sliding) friction — the resistance when the surfaces are actually sliding over each other; usually a little less than the limiting static value.
• Rolling friction — the (much smaller) resistance experienced when a body such as a wheel or ball rolls over a surface.
• Fluid/film friction — the resistance arising in a lubricating film separating the surfaces.

By contrast, gravitational friction is not a recognised type of friction, so it is the odd one out and the correct answer. Gravity is a body force acting at a distance; it can produce the normal reaction that presses surfaces together (and thereby influences the friction force through F = μN), but gravity itself is not a contact resistance and does not constitute a class of friction.

• Static friction, kinetic friction, and rolling friction are all standard, well-defined types, so none of them is the answer.

Thus the term that is not a type of friction is gravitational friction.