An Etymological Dictionary of Astronomy and Astrophysics

A dimensionless quantity characterizing the → interstellar extinction, defined by the total-to-selective extinction ratio:
R_V = A_V/E(B-V). The typical value found for the reddening parameter in the Milky Way is R_V ~ 3.1, but it is known to vary from one
line of sight to another, from values as 2 to as large as 6. Very large → dust grains would produce extinction with R_V → ∞.

See also: → reddening; → parameter.