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, "Louis believes x is a hero" is an opaque context because "Louis believes Superman is a hero" is true while "Louis believes Clark Kent is a hero" is false, even though 'Superman' and 'Clark Kent' are co-referential expressions.