UI is short for "User Interface" and refers, in our context, to the design and interactions of a website or app, or more general, to how the user interacts with your site or app. UI Design is the creation of the user interface, by designing and building it.
What is a UI (User Interface)?
From the Responsive Design Glossary
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.
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