Senior officer
A senior officer or field officer was any commissioned officer that was not a commanding officer or flag officer, who served as 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 the Earth Starfleet, where there were limited personnel choices with special talents, junior officers by rank were sometimes senior officers by practicality. Some such personnel included:

Ensign Travis Mayweather, pilot of , experience with other worlds, as part of the Earth Cargo Service
Ensign Hoshi Sato, communications officer of Enterprise, exceptional linguistic abilities
Lieutenant Malcolm Reed, armory officer on Enterprise, chief of security

In the Federation Starfleet, senior officers usually included officers with the rank lieutenant commander or above and could include, but was 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 could also add certain officers at his or her discretion. These officers had the privilege of sitting in with the senior staff during briefings, or at times had the ear of the commanding officer.

Some of the more permanent additions have included: 

Commander Deanna Troi, ship's counselor,  / 
 Tom Paris, flight control officer, 
Seven of Nine, astrometrics laboratory supervisor, USS Voyager
Ensign Harry Kim, operations officer, USS Voyager

Occasionally, one was referred to as a senior officer in a manner that was equivalent to a ranking officer. In 2368, Counselor Troi, who at the time was a lieutenant commander, was described as the senior officer on the bridge deck of the Enterprise after it struck several quantum filaments. () Another occasion occurred in 2369, when Montgomery Scott and Geordi La Forge were working together on the  to save the Enterprise-D. After they got the vessel's engines back online, Scott told La Forge to take the bridge. La Forge initially refused, replying that Scott was the senior officer, but Scott told La Forge that he was a Captain by rank, but that he had never wanted to be anything other than an engineer. () In 2371, 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. () In 2373, Lt. Commander Jadzia Dax referred to herself as a "simple field officer," when confronting Captain Benjamin Sisko about his and the "brass hats" at Starfleet Command's recent complacency in the Dominion War. () In Tuvok's Insurrection Alpha holographic program, Commander Chakotay planned to discard all Starfleet senior officers at the first habitable planet that Voyager came across following a Maquis mutiny. Those of lower status had the choice to either join him or also be left on the planet. ()

In the Imperial , senior officers lived in fear of the day when they were assassinated by an ambitious subordinate, seeking promotion to a higher rank, and were, thus, in a constant state of suspicion. (; )

External links
 
 

cs:Vyšší důstojník
de:Führungsoffizier
ja:上級士官
nl:Hoofdofficier
fr:Officier senior
Titles
Mirror universe
Military ranks