Usability is generally understood as the degree of user-friendliness of a product for satisfactorily achieving individual goals.
Usually, the products are software, website, mobile devices, medical machines or machine control systems which are used by humans. The usability is usually intuitive, depending on whether the product is expedient or not. There are various methods for measuring the usability, for example eye tracking.
The term web usability describes the specific user-friendliness of a website. If a website is set up in a user-friendly way, the visitor effectively reaches his destination, for example when booking a trip. In this case the likelihood of him visiting the site again increases.
Usability is closely related to the term user experience (UX). This extends usability in terms of emotionality and aesthetics.