A distinction between the contexts into which referring expressions can be put. A context is referentially transparent if any two terms referring to the same thing can be substituted in it salva veritate, i.e. without altering the truth or falsity of what is said. A context is referentially opaque when this is not so. Thus, if the number of the planets is nine, then ‘the number of the planets is odd’ has the same truth-value as ‘nine is odd’; whereas ‘necessarily the number of planets is odd’ or ‘x knows that the number of planets is odd’ need not have the same truth-value as ‘necessarily nine is odd’ or ‘x knows that nine is odd’. So while ‘…is odd’ provides a transparent context, ‘necessarily…is odd’ and ‘x knows that…is odd’ do not. See also extension/intension.