The Oort constants are two constants, the result of measured values of the angular velocities and distances to stars in the solar neighborhood, including a determination of the Sun's position and velocity. The two constants characterize the rotation curve of the solar neighborhood (local region of the Milky Way), and can be used in calculating distances and velocities of stars in certain directions (not too close to or far from the galactic center) and distances (not across to the far side of the galaxy), given some measurements.
The two constants are known as A and B and modern determinations see:
These values indicate a nearly flat rotation curve over the regime described above, implying "there must exist dark matter".
Example formulae making use of them:
Vr = A d sin(2l) Vt = A d cos(2l) + Bd