UX is short for "User Experience" and is the overall experience of a user using a product or interacting with a website. The focus is on making a good user experience, where interacting with a website is pleasing and easy. A good design, clear content, fast load times and nice interactions with appropriate animations are all part of a good user experience. In common usage, "the UX" of a site or app often refers to the design and interaction combined.
The responsive design glossary explains the many terms used in responsive design and development. It's a practical reference for anyone getting started with responsive design and development.Get an overview of all responsive design terms