Senior officer

From Memory Alpha, the free Star Trek reference.

A senior officer denotes a member of the senior staff of a starship or starbase involved in the top level, command decisions of the day-to-day operations of their post. In Starfleet, this includes officers with the rank Lieutenant Commander or above and can include, but is not limited to (regardless of rank):

• Commanding Officer
• Executive Officer
• Operations Manager
• Chief Engineer
• Chief of Security
• Chief Medical Officer
• Science Officer

The commanding officer can also add certain officers at his or her leisure. These officers have the luxury of sitting in with the senior staff during briefings, or at times can hold the ear of the commanding officer.

Some of the more permanent additions have included:

• Commander Deanna Troi, Ship's Counselor, USS Enterprise-D/USS Enterprise-E
• Lieutenant Tom Paris, Flight controller, USS Voyager
• Seven of Nine, former Borg drone/Astrometrics Laboratory Supervisor, USS Voyager

Occasionally, one is referred to as a senior officer in a manner that is equivalent to a ranking officer. Both B'Elanna Torres and Tuvok tried to take responsibility as the senior officer in the installation of a spatial trajector without Captain Kathryn Janeway's consent. (VOY: "Prime Factors")