Feedback
Feedback is when something reacts to the user, giving them some indication that an action occurred. In the context of user interfaces and websites, this usually means styling effects that help a user to understand how to navigate the interface.
Common example of feedback
A good example of feedback can be seen in the design of buttons that users can click on, such as this.
There are three main aspects of feedback going on here, that help the user to understand that this is a button, and not just a rectangle with text:
Colour change and transition
text
The border imitates a real-world, 3D button
text
Cursor change
text