Affordances
Define what actions are possible with an object or interface based on the capabilities of the user.
Affordances help you know what action to take.
Help users achieve their goal by connecting what an object does according to their Conceptual Model of what that object should do.
The form of the interactive element implies the function
Affordances help people figure out what actions are possible without the need for labels or instructions. For example, buttons afford pressing because they often appear to be raised from the surface, or they are styled differently from other content often in addition to text that implies an action.
Distinction: signifier vs affordance
Affordances are the actions that are possible, while signifiers communicate where the action should take place.
Origin
This fundamental principle of interaction was defined by researcher, professor and author Don Norman and can be applied to make products and services efficient, effective and delightful to use. It results from the appropriate application of five fundamental psychological concepts: affordances, signifiers, constraints, mappings and feedback.