An opaque context or referentially opaque context is a linguistic context in which it is not always possible to substitute "co-referential" expressions (expressions referring to the same object) without altering the truth of sentences. The expressions involved are usually grammatically singular terms. So, substitution of co-referential expressions into an opaque context does not always preserve truth. For example, "Lois believes x is a hero" is an opaque context because "Lois believes Superman is a hero" is true while "Lois believes Clark Kent is a hero" is false, even though 'Superman' and 'Clark Kent' are co-referential expressions.

Usage

The term is used in philosophical theories of reference, and is to be contrasted with referentially transparent context. In rough outline: Similar usage of the term applies for artificial languages such as programming languages and logics. The Cicero–Tully example above can be easily adapted. Use the notation [t] as a quotation that mentions a term t. Define a predicate L which is true for terms six letters long. Then [x] induces an opaque context, or is referentially opaque, because L([Cicero]) is true while L([Tully]) is false. Programming languages often have richer semantics than logics' semantics of truth and falsity, and so an operator such as [x] may fail to be referentially transparent for other reasons as well.