There are umpteen definitions available on the internet for User experience(UX) design. Here are some of my personal favorites:
“User experience (UX) design is the process design teams use to create products that provide meaningful and relevant experiences to users.”
Nielsen Norman Group
"User experience" encompasses all aspects of the end-user's interaction with the company, its services, and its products.”
I agree with all of the above definitions of User experience design. To make it even simple, I would define User experience design as
“Designing for the users.”
It can be an interface, mobile app, desktop website, or a physical product. The design should always put the users at the center.
One of the common misconceptions is that design is the look and feel of the product, but it is much more than that. Design is about how the product works and how usable the product is.
Here is a famous quote from the co-founder of Apple, Steve Jobs:
“The design is not just what it looks like and feels like. The design is how it works”
— Steve Jobs.
Check out this interesting video regarding the term UX from the pioneer in the field of user experience design and author of several books on user experience design, including “The Design of Everyday Things.”