[…] we tentatively understand the notion "variable" in the following sense, subject to later refinement: (4) α is a variable if and only if α = [NP "e"] in S bound by an operator.
- Chomsky (1993), Pag. 102

[…] [NP e] is a variable if and only if it has Case.
- Chomsky (1993), Pag. 175

[…] α is a variable if and only if (i) α = [NP e]; (ii) α is in an A-position (hence bears an A-GF); (iii) there is a β that locally Ᾱ-binds α.
- Chomsky (1993), Pag. 185

If α is an anaphor or lacks a phonetic matrix, then (i) α is a variable or (ii) α is bound in every governing category.
- Chomsky (1993), Pag. 225

[...] α is a "variable" if and only if it is locally Ᾱ-bound and in an A-position.
- Chomsky (1993), Pag. 330