Every design problem begins with an effort to achieve fitness between two entities: the form in question and its context. The form is the solution to the problem; the context defines the problem. We want to put the context and the form into effortless contact or frictionless coexistence, i.e., we want to find a good fit. - Christopher Alexander

Terminology in use

Interaction Design (IxD); User Experience Design (UXD);Usability Engineering (UE); Human Computer Interaction (HCI); 
Computer Human Interaction (CHI); User Interface Design (UID);