There are multiple terms, that describe UX design activity. They include both UX and such terms as UI design, Interaction design, Usability, Human factors, Ergonomics and so on.
We will use the term UX design (or simply UX), just because it is most popular and describes the activity in most general (and straightforward) way — as design of user experience with interactive product. So:
User experience design is an activity to design products to allow the users to achieve their goals and complete their tasks by using the product in most effective and simple way — with less efforts, time and errors.
This definition is not so bad, but requires specifics. Let’s discuss it in details.
Users — are the humans, that use the product to achieve their goals or resolve their tasks. In more specific contexts users are also called as operators.
Product — is software application that is running on hardware, or combines them both. In first case, the hardware is general, like desktop PC, laptop or smartphone and the product is installed as software (but it can impose specific hardware requirements). In the second case, especially for professional equipment — the hardware is specially created for product and is the inevitable part of it — like medical ultrasound scanner or professional photocamera. Also, the product can combine several computers, making more complex interactive system — like the ones in air traffic control centers.
Usage of the product is actually an interaction between the user and the product. The user affects the product input devices with his muscles (by pressing the buttons or touch screen, by moving the mouse or trackball, by rotating the wheel), and receives the response from it by watching on the display or other output devices.
All this is not done in void space, but in some real context. The context includes both the environment, in which the user interacts with it (like office room, ship deck, outdoors, etc.), and the subject domain of interaction itself — i.e. navigating the ship, making medical research, creating the plan of future building and so on.
What is also important for professional products, typically the users don’t start to use it immediately — distinct time for learning the system is required.
Next: Interaction loop