Word: Signifier

Definition: A signal that communicates appropriate behavior to a person and where an action should take place.

Reference: Don Norman explains the subtle difference between signifiers and affordances in his book, "The Design of Everyday Things". He amended the book in later editions because while the design community took fondly to the term affordance, Norman believed it was being improperly used (based on his definition).

Thoughts: Touch screens afford to touch... but as to where and how to touch, this is a signifier.