affordance

Anything that is provided or furnished by an environment to an organism dwelling within it.

So an affordance cannot be measured as we measure in physics.

A potential transaction or operation that is made possible by a given object or environment; especially, one that is made easily discoverable.

Take for instance a paperback book. It has a wide variety of affordances including conveying a narrative story, weighing down papers in a windy office, swatting houseflies, etc. While these affordances are properties of the book, whether they are available depends on the context.

Any interactive control or component serving as a cue to the user to take some action.

The grab handles on the edges of this image are affordances that the user can exploit to change the size of the image.

Affordances provide strong clues to the operations of things. Plates are for pushing. Knobs are for turning. Slots are for inserting things into. Balls are for throwing or bouncing. When affordances are taken advantage of, the user knows what to do by just looking: no pictures, label, or instruction is required.