Usability is the ease of use and learnability of a human-made object.
The object of use can be a software application, website, book, tool, machine, process, or anything a human interacts with. It is widely used in consumer electronics, communication, and knowledge transfer objects (such as a cookbook, a document or online help) and mechanical objects such as a door handle or a hammer.
Web usability is the ease of use of a website. Some broad goals of usability are the presentation of information and choices in a clear and concise way, a lack of ambiguity and the placement of important items in appropriate areas. One important element of web usability is ensuring that the content works on various devices and browsers. Another concern for usability is ensuring that the website is appropriate for all ages and genders.
A userfriendly, intuitive interface design is the core element of the web usability, which is strongly related with information architecture (implemeted as easy-to-use navigation), interaction design (anticipating the user’s behaviour) as well as the user’s experience (which is the expected result of the before mentioned topics, when well applied).
Web usability can be tested scientifically with eye tracking technology and variousstatistical research, but in real life web productions, it is, similar to interaction design, more about imagining things. A usability designer must be able to switch in the user’s role to know his behaviours and preferences and to satisfy their needs. It’s the key player when it comes to create positive user experiences.
More about usability at usability.gov.