Definition
Referring expressions, R-expression for short, generally refer to an NP that gets its meaning by referring to an entity in the world. (Carnie, 2006, p. 147)
In Halq’eméylem, independent pronouns are R-expressions and these expressions are subject to Condition C effects when in argument positions. (Lai, 1998, p. 36)
In Secwepmctsín, independent pronouns attached to a lexical determiner phrase (DP) are R-expressions and are subject to Condition C effects. (Lai, 1998, p. 30)
Examples

References
Carnie, A. (2006). Syntax: A generative introduction. John Wiley & Sons.