Operator

Operator is element, such as a quantifier (i) or a wh-phrase (ii), which A-bar binds a variable (x) at LF (see A-bar binding):

(i) Mary loves someone LF: someone x, Mary loves x (ii) Who does Mary love? LF: for which x, Mary loves x

Links
Utrecht Lexicon of Linguistics

Semantics
a logical constant (O) which is (usually) prefixed to a formula phi in order to produce a new formula Ophi. The interpretation of this formula is a compositional function of the interpretation of phi. Negation is a truth-functional operator, which operates on the truth value of the proposition it is combined with. Modal operators (like necessarily) and tense operators (like it has been the case that) are not truth functional, because their interpretation does not solely depend on the truth value of the formula with which they combine, but also on the possible world or moment of time with respect to which the whole sentence is evaluated.

Links
Utrecht Lexicon of Linguistics