UI

UI stands for User Interface which in computer terms usually refers to anything you might interact with as a user on the screen. Most often when web developers say "UI" or "User Interface" they're referring to the design of a website.

Technically, a GUI (Graphical User Interface) is a more accurate term for the design of a website. GUI's are types of UI's that have a graphical aspect to them. They are a sub-category of UI. In other words all GUI's are UI's but you can have a UI that is not a GUI, such as a Command-Line Interface.

As we stated before, in our industry most people just say UI when they refer to design. For example:

Hey, I need you to send me the UI so I can start on the HTML for this project.