R-expression is an element whose reference cannot usually be determined (but see anaphoric epithet) anaphorically in the sense of the binding theory and as such is distinguished from anaphors and pronominals. R-expressions are also distinguished from quantificational noun phrases and predicative NPs in that they must be in an A-position in LF. With respect to A-binding, variables - either wh-traces or traces of QR - count as R-expressions. R-expressions are not to be confused with referential expressions. With respect to binding theory, R-expressions obey condition C, which says that they must be A-free. This explains the illformedness of (ii) and (iii) (see Strong Crossover).
(ii) * hei thinks that Johni is a fool (iii) * whoi does hei like ti