Nuclear Stellar Disc (NSD)
Not present in the crianca summary, however, the analogy for Stellar Proper Motions is: imagine you're watching a group of kids playing tag in a park. Each kid is like a star, moving around in the park. If you know how fast each kid is moving and where they are going, you can figure out how far away they are from you.
Technical definition
A disk-like structure surrounding the supermassive black hole at the center of the galaxy, comprising a population of stars.