To say something is usable is a qualitative statement about how easy that thing is to use. Usability is an assessment of how learnable a system is and how easy a user finds it to use. The usability of a system or product is a key factor in determining whether the user experience is a good one.
To make a system usable one must remove friction from the experience, reduce any potential learning curves and foster a feeling of satisfaction in completing any tasks.
The core variables in judging the usability of a system include who the users are, what their goals are and the context of why they are using the system. The effectiveness, efficiency and satisfaction derived from the experience can be assessed to determine a general level of usability.
Designers creating interactive systems should have a good understanding of basic usability principles in their medium in order to design a usable baseline that can be tested and observed to iterate and improve upon in order to increase the usability.
Usability is a practice that can be performed by people from many disciplines.