Phonological opacity

Phonological opacity is a term used in phonology. It was first defined by Kiparsky as a measure of how far the context or the consequences of a phonological process may be determined only by examining the surface structure. Kiparsky defined it in the following way:
A phonological rule P, , is opaque if any of the following surface structures exists:

• instance of A in the environment;
• instance of B created by P in an environment other than ;
• instance of B not derived from P that occur in the context .