Commanding officer

A commanding officer or CO was, according to Ben Jackson, "the boss" of a military installation. The First Doctor seemed unfamiliar with the term, and therefore had to get Ben to translate it for him. (TV: The Tenth Planet)

History
There was a CO at the American Army Air Corps base in Foggia, Italy during the First World War. (PROSE: Have You Seen This Man?)

The leader of the British division of UNIT was known as the commanding officer. (AUDIO: Time Heals) Brigadier Alistair Gordon Lethbridge-Stewart was the CO from its inception. (TV: The Invasion) After his retirement, Colonel Charles Crichton took over. (TV: The Five Doctors)

By 1997, (TV: Battlefield) Brigadier Crichton had transferred to the Forge (PROSE: Project: Valhalla) Brigadier Winifred Bambera took over (TV: Battlefield) until at least 2001. (AUDIO: Animal) Although she stopped being commanding officer, she remained at UNIT until at least the 2020s, by which point she was a general. (AUDIO: Signs and Wonders)

Colonel Ross Brimmicombe-Wood was commanding officer by 2005. After his disappearance, Colonel Robert Dalton stood in for him (AUDIO: Time Heals) until his death. Following Brimmicombe-Wood's arrest, Colonel Emily Chaudhry became CO. (AUDIO: The Wasting)

Colonel Mace was CO by 2009. (TV: The Sontaran Stratagem/The Poison Sky) He was stationed in Vancouver between at least September 2009 (TV: Children of Earth: Day One) and June 2010. (PROSE: The Forgotten Army)

Chief scientific officer Kate Stewart was leading UNIT by 2013. Unlike her predecessors, she was not military. (TV: The Power of Three, The Day of the Doctor)