UX is User Experience.
The user experience is how a user interacts with and experiences a product, system or service. It includes a person’s perceptions of utility, ease of use, and efficiency.
-Wikipedia
UX is not just about wireframes or mockups. It refers to the entire experience, whether it’s a website, an app or a physical space. It includes everything from colors, graphics, typography, smells, sounds, interactions, conversations, textures, data visualizations and more. A good user experience delights you makes you want to participate and return again. A bad user experience causes frustration and makes you want to leave and not return.
In terms of software applications, websites and mobile experiences, the definition gets more specific; we talk about usability, functionality, performance, accessibility and and other criteria, depending on the purpose of the product.
What is good UX?
Can a user accomplish whatever it is they are trying to do? Beyond just getting things done, how was the experience? Was it complicated or tedious? Did it delight the user? Or was it just adequate?