The term "proper motion" derives from the historical use of "proper" to mean "belonging to" (cf, "propre" in French), so there is no such thing as "improper motion" in astronomy.
The term "proper motion" derives from the historical use of "proper" to mean "belonging to" (cf, "propre" in French and the common English word "property"). There is no such thing as "improper motion" in astronomy.