Senior officer/Legends
The title of senior officer was in use in the Imperial Military, apparently as a designation for the commanding officers of small, elite deployments, in forces such as the Stormtrooper Corps and Imperial Security Bureau. Though the term may also have been used in other contexts, it is here that it is best recorded.
Officers using this title included stormtrooper DSS-0956 (apparently a captain), who commanded the Desert Sands stormtrooper unit aboard Darth Vader's Star Destroyer Devastator; and Colonel Vak Somoril, who led the ISB tactical unit assigned aboard the Star Destroyer Reprisal in 0 ABY.
Behind the scenes
Galaxy Guide 1: A New Hope refers to the commander of the twenty-man stormtrooper detail on Tatooine as "Desert Sands senior officer DSS-0956" in the text and in the heading for his character template. In the dramatis personae list for Allegiance, Vak Somoril, commanding an ISB unit of around a hundred men, is called "senior officer, Imperial Security Bureau", rather than by his rank of colonel, where other characters are given specific titles like "captain", "general", "director of planning" and "Supreme Commander".