An adjutant serves as an aide to a more senior officer. An adjutant can rank anywhere from an ensign to captain.
In 2270, Yeoman Ferro and Lieutenant Pavel Chekov acted as adjutants to Captain James T. Kirk during a high-level briefing at Starbase 6. (TOS - Unlimited comic: "Action of the Tiger")
During the Dominion War, Captain Benjamin Sisko acted as an adjutant to Vice Admiral William Ross. (DS9 episode: "Behind the Lines")