The term uniform was used to refer to the clothing worn for the purpose of displaying membership in an organization. During the Trojan War, Steven Taylor noted that, by taking a uniform, he would be captured as a prisoner of war instead of a spy. (TV: The Myth Makers)
During the 2000s, red caps constituted part of the uniform of UNIT soldiers. (TV: The Sontaran Stratagem/The Poison Sky) Reuniting with his old friend Martha Jones, now a member of UNIT, Jack Harkness jokingly asked if she could provide him with a red cap, suggesting that Ianto Jones would look good in one. Martha told Jack to get his own uniform. (TV: Reset) Bilis Manger noted how Jack was "not in uniform" when encountering him in 1940s war-time Britain, with Jack using the excuse that he was on leave for 48 hours. (TV: Captain Jack Harkness)