What is Usability?

Usability is simply pertinence, which is logical precise relevance by virtue of being applicable to a unique product of technology - a web site, a mobile device, software, hardware, or any other product of technology. It encompasses:

• applicability
• appropriateness
• adaptation
• suitability
• fitness

• The relevance of the system to serve the user's needs.
• The efficiency of carrying out tasks using the system.
• The user's attitude to the system and their subjective feelings.
• The learnability of the system.
• The safety of the system.

Quoting ISO [2], the term usability is defined as:

"the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction in a specified context of use"

Despite of those neatly mentioned definitions, one may wonder the use of them in practice. There is no "usability science", instead the term "usability engineering" is more commonly used by usability practitioners and specialists. From the practical point of view, the definitions are only useful for the delimitation of scope, focus, and bias of the usability practice.

References

1. Paterno, Fabio (2000). Model-based design and evaluation of interactive applications, chapter 8. p.141-175. Springer-Verlag London Limited, 2000.
2. ISO (1998). ISO DIS 9241-11. Geneve