proper motion

WordReference Random House Unabridged Dictionary of American English © 2025

prop′er mo′tion, 

1. Astronomy the angular motion of a star relative to a suitably defined frame of reference, expressed in seconds of arc per year. Cf. tangential motion. 

• 1595–1605

Collins Concise English Dictionary © HarperCollins Publishers::

proper motion noun

1. the very small continuous change in the direction of motion of a star relative to the sun. It is determined from its radial and tangential motion