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Variable

SYNTAX: an empty element, such as a wh-trace, or a trace of QR, which must be A-bar bound by an operator. A variable counts as an R-expression with respect to principle C of the Binding Theory.
LIT. Chomsky, N. (1981)

SEMANTICS: a basic expression of a logical language which is used as a place-holder in a formula. It does not have a specific reference but stands for an unspecified value. In first order predicate logic, individual variables ranging over individuals are the only variables used, but in higher order logic and type logic variables can also range over sets, relations and functions of diverse complexity.
LIT. Gamut, L.T.F. (1991)