Definition of Usability

ISO defines usability 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.” (ISO 9241-11)

• 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.

According to Jakob Nielsen, usability is a quality attribute that assesses how easy user interfaces are to use. He saids the word “usability” also refers to methods for improving ease-of-use during the design process. He definded usability by 5 quality components.

• Learnability :
  How easy is it for users to accomplish basic tasks the first time they encounter the design?
• Efficiency :
  Once users have learned the design, how quickly can they perform tasks?
• Memorability :
  When users return to the design after a period of not using it, how easily can they reestablish proficiency?
• Satisfaction :
  How pleasant is it to use the design?