UX + Instructional Design=UXD.
UX Design studies user interactions with a service, a product, or even a concept.
That being said, instructional design (ID) can be very close to UX. Indeed, ID is designing content, lessons, courses, exercises,… for a user, the learner.
But things are not so simple…
UX and ID are close, but their subject diverge.
A user is not a learner.
As Niels Floore said, « A user wants to watch a movie, a learner wants to speak a foreign language« .
And a teacher is not the learner. That means you have to know about your learners, and not presume their needs and motivations, before knowing them.
UX and Instructional designers are friends, they can influence themselves, but ID is, however, a very specific and complex field of study.
UXD is mixing user interaction design models applied to designing learning content.