Common Principles: A Usable Interface Design Primer

When users perform a transaction or action, their cognition is often split between learning and operating the system or user interface (UI). A well-designed UI allows users to focus the majority of their cognitive energy on learning, and offers no operational complications. This most general principle of usability is often called the 'transparent interface.' The transparent interface is commonly defined as one that maximizes user task completion and minimizes interfering factors, such as unnecessary interface complexity or performance.

Oppedisano, Rick. Usability Professionals Association (2002). Design>User Interface>Usability