Defining user experience design is pretty straightforward. Our life is a linear series of events. Things we had never heard of slide into our awareness. Maybe someone tells us about something, maybe we walk around and see something or someone, etc. You brain tries to make sense of it based on existing experiences, familiar patterns, etc. Via more exposure, you form a better understanding.

Apps are the same. Someone hears about it and wants to figure out generally what it's for. They see it and form a split-second opinion based on initial presentation. They decide to read about the features/benefits. They download it and use it, etc. At each step information is gathered and decisions are made. Maybe they get it right away, maybe they struggle. Maybe this happens in 1 minute, or over 2 months, etc.

As a user experience designer, you consider this general sequence of events, and the many ways people can experience it across all possible touch points, and try to create smooth pathways for people. And you typically have to balance user needs & wants, tech possibilities and limitations, business needs, human psychology, available resources, and other stuff. It's a huge balancing act that benefits from tight collaboration.

YMMV