What is "retardance"

1550s, "retardation," from Middle French retardance, from retarder (see retard (v.)). In reference to resistance to fire, 1948.

n. (context physics English) The difference in phase shift between two characteristic polarizations of light upon reflection from an interface. Retardance can be reported in the Abelès or Mueller conventions. The retardance phase shift angle is often specified in degrees.