User Xperience Design is developing a solution that makes a user be rather with the solution than without it. You can not find a more precise definition as an UX-Designer works in multiple different fields - as shown here.
Usability is defined by how easy it is to use the solution that was developed for a specific problem. It can be measured by multiple techniques like user-testing with different standardised questionnaires.start with Usability
Part of a good user experience is a usable visual Design. That means that certain elements of the UI have a certain affordance and a user knows intuitively what they are designed for.find out more
Another big topic for a UX designer is IT. It is essential to knwo how to implement code. This knowledge can be used to implement prototypes with a higher functionality but also helps to be the connection between designers and developers.find out more
As User Xperience Design has to do a lot with the entire experience of the product aspects like ergonomics are essential for the design of products or the packaging.find out more
As you always work together with and for the users is it is necessary to have some knowledge in the field of psychology to understand the users' behaviour better.find out more
Obviously as an UX designer it is not required to be a marketing expert. You do, however, need knowledge on that field, too, as this will also be part of the user experience.find out more