The things people see in your application. Anything that can be captured in screenshots. Note that this excludes the behavior of your application (how responsive is it?) and doesn't take into account the mental state of the user. What are they trying to achieve? Does the application make their journey a delight or a torment? To answer these questions we move to the richer concept of UX.
My book "Choose Your First Product" is available now.
It gives you 4 easy steps to find and validate a humble product idea.