Usability is defined by ISO/IEC 25010:2011 SQuaRE -- System and Software Quality Models as the degree to which a product or system can be used by Stakeholder (i.e., specified users) to achieve specified goals within a specified context.
The goals of usability are1):
This characteristic is composed of the following sub-characteristics2):
Usability as a characteristic is often considered a subjective quality and left to “interpretation”, however, there are metrics which use to quantify these sub-characteristics. Before we delve into the definition of the specific metrics, it is important to understand why we need metrics rather than just rely on intuitive evaluations.
A core reason to collect Usability Metrics is to provide data about a stakeholder's understanding of a product's usability rather than the developer's understanding of usability. When the two understandings (i.e., interpretations) converge everyone is happy resulting in a way forward. That result may be to either continue in the same direction or to have a reassessment of the user's needs.
The metrics must quantify that the system meets the goals of the overall system:
Ultimately, the primary objective of usability metrics for evaluating a system or product is properly engineered (i.e., neither under- or over-engineered).