Found:

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 variable**s 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) |