An Etymological Dictionary of Astronomy and Astrophysics

1. The state of being → opalescent, or emitting an → iridescence like that of an opal.
   

2. Physics: The milky → iridescent appearance of a dense transparent medium when it is illuminated by → polychromatic visible radiation (such as sunlight) due to local → fluctuations in its density and therefore in its → refractive index. → critical opalescence.

See also: → opal; → -escence.