UX
Also known as: User experience
UX is the overall experience a person has using a product, covering how easily and successfully they achieve what they came to do.
In full
User experience design concerns the whole of a person's interaction with a product: whether they can find what they need, understand what they are looking at, complete the task they arrived for, and recover when something goes wrong. It encompasses information architecture, interaction design, content structure, and the flow between steps. UX is distinct from UI, user interface design, which concerns the visual and interactive surface itself. A site can be visually accomplished and still fail on UX if navigation is confusing, forms are punishing, or the path to the thing people actually want is buried.
Why it matters
Most websites that fail to convert do not fail because of their appearance. They fail because the offer is unclear, the proof is missing, or the path to enquiry takes too many steps.