proper motion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gradual change in the position of a star or other object on the celestial sphere that is the result of the object's intrinsic motion through space rather than mere apparent motion as observed from Earth.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy The change in apparent position of a celestial body due to that body's velocity relative to the Sun.