An adjutant (from the Latin adiutans, present participle of the verb adiutare, "to help"; the Romans actually used adiutor for the noun) is an officer who assists a more senior officer.
Commander Erika Benteen was adjutant to Vice Admiral James Leyton in the 2370s. (DS9: "Homefront", "Paradise Lost")
Richard Boswell served as deputy and senior adjutant to Lawrence Cheung during Boswell's time aboard Starbase 505. (Star Trek: Remington)