Just found a good term “learning science”, which I think better describes the “to be discovered” scientific fundamental of learning, especially human learning.

Instructional design, or pedagogy then apply the discoveries of learning science to achieve the instructional goal. Note that learning science does not have goal but instructional design must contain. Instructional design is the “engineering”. Thus it is clear there is a R&D flow from learning science to instructional design, but note that discoveries during instructional design and teaching can be feedback to learning science too. It is not one way but reinforcing loop that potentially greatly advance our understanding of learning and improve the education we desired.