colour excess BrE
color excess AmE

In the UBV photometric system the color excess is related to the B-V colour by:

In any photometric system interstellar reddening can be described by color excess, defined as the difference between an objects observed color index and its intrinsic color index (sometimes referred to as its normal color index).

The amount of reddening is characterized by color excess, defined as the difference between the Observed color index and the Normal color index (or Intrinsic color index), the hypothetical true color index of the star, unaffected by extinction.