Empty category

Any node without phonological content. Usually indicated as e or ec. Empty categories come in different varieties, depending on their syntactic properties, such as Case, Theta-role and (potential) antecedent(s). NP-traces have no Case, whereas wh-traces may have Case (also see trace). The trace of an adjunct has no theta-role and is A-bar bound. An NP-trace has a theta-role and is A-bound. Empty categories can be the result of deletion or movement, or they are base-generated, viz. pro and PRO. Different types of empty categories behave differently with respect to the Empty category principle and binding theory.

Link
Utrecht Lexicon of Linguistics