Usability is an umbrella term encompassing ease of use, learnability, quick error recovery, and support of a range of defined users, from novices to experts. It is more formally defined by the Institute of Electrical and Electronics Engineers (IEEE) as "the ease with which a user can learn to operate, prepare inputs for, and interpret outputs of a system or component" (IEEE, 1990). At its most basic level, a usable product facilitates achievement of the user's goals without making it difficult for the user to reach those goals.

The International Organization for Standardization (ISO) is a centralized network for standards organizations across the globe, which began in the electrotechnical field in 1906 and has now extended to a broad range of engineering and technology areas (ISO, 2005). The ISO 9000 standards concerns quality-management techniques intended to improve customer satisfaction. In standard 9241, ISO defines "Ergonomic requirements for office work with visual display terminals (VDTs)" (ISO, 1997), including in that standard a definition of usability. Volker Schöch (1999) extracted from the ISO 9241 standard the following plain language definitions of usability, and related terms:

• Usability - The effectiveness, efficiency, and satisfaction with which specified users achieve specified goals in particular environments.
• Effectiveness - The accuracy and completeness with which specified users can achieve specified goals in particular environments.
• Efficiency - The resources expended in relation to the accuracy and completeness of goals achieved.
• Satisfaction - The comfort and acceptability of the work system to its users and other people affected by its use.

Such a definition leads to design objectives and finally provides the means for explicit measurements for usability.

While clearly related to ergonomics, usability is a comparatively new concept, having emerged tentatively in the late 1980s, and having moved into more extensive use in the 1990s. The set of criteria covered by the umbrella term evolved along with the design adaptation of computers and software for widespread use, although it is now also applied to areas of design and human interaction that overlap with equipment and products addressed by the more established field of ergonomics. The science of ergonomics was not readily applied to software design for two reasons: (1) how and why software evolved as a widespread, human work tool; and (2) the complexity and maturity of the ergonomics field and its having developed into specialties by the time software emerged as an independent product. Further, usability is still largely in a state of practice, not yet having evolved into an established science. While the numbers and quality of studies on usability are increasing, empirical literature in the field remains sparse.

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