UX debt is the quality gap between the experience your digital product delivers now and the improved experience it could offer given the necessary time and resources. Put another way, UX debt measures the number and magnitude of potential product enhancements that would improve the user experience.

In his book Designing for Emotion, Aarron Walter remapped Maslow's Hierarchy of needs to the needs of our users. To paraphrase Mr. Walter: for a user's needs to be met at the most basic level, an interface must be functional. If you can't complete your task, you won't stick around long. Each level of the “functional, reliable, usable, pleasurable” pyramid satisfies different user needs and increases a user’s satisfaction with a digital product. UX debt is measured as the distance between the current quality of experience and the target quality.