Usability [1] means to make users feel more effective, efficient and satisfied when using a product or system. According to the international standard, ISO 9241-11, 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.

For instance, in Internet website design, we would like to let users not to have any pressure or frustration in web surfing and allow users to use a minimum effort for gaining the maximum performance during their using the site function.
General design guidelines for usability
[2] Usability expert Jakob Nielsen and computer science professor Ben Shneiderman have separately commented about the acceptable degree of the framework of a system. It is stated that usability is part of the "helpfulness" and it contains the following elements:

 Learnability: The measure of how a user can effectively use a product or system in the first time.
 Efficiency: The speed that a user learns how to practice the system and perform the task.
 Memorability: The design can be navigated and guide the user to seek what they look for, furthermore, users can recognize and remember when they return.
 Errors: How many errors do users make, how severe are these errors, and how easily can they be recovered from the errors?
 Satisfaction: How pleasurable is it to use the product or system?

Nowadays, users always control the system by user interface, thus a good interface affects the usability of a good system. There are many good user interface design definitions on the internet and we can summarize them in seven laws[3].

1. Law of clarity: Prevent the interface from having no clear meaning as the human nature would lead users to ignore the things which they do not understand.
2. Law of preferred action: Users would feel comfortable when they know what the preferred action is.
The following diagram...

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