9126-1 Quality Model: Usability
3. A set of attributes related to the effort needed for use, and to the individual assessment of such use, by a stated or implied set of users.
3.a Understandability, attributes related to the users' effort for recognizing the logical concept of a software system and its applicability.
3.b Learnability, attributes related to the users' effort for learning its application (for example, operation control, input, output).