Starting in the 1960s, as The Doctor had more encounters on Earth, he encountered on occasion some military men that could help with his missions. One of them was Alistair Gorgon Lethbridge-Stewart, and his encounter with him and his men involving a Yeti led to the formation of a new agency, tasked with the defense against extraterrestrial threats. This agency would be called the Unified Intelligence Taskforce (formerly the United Nations Intelligence Taskforce), or UNIT, with Lethbridge-Stewart as its first commanding officer.
This organization would be the formal face of defense against alien threat, growing from humble roots in the 1960s to a fully fledged defensive agency for the new millennium. Although The Doctor was one not to use physical or weapon-based threat, when the situation was needed, the personnel of UNIT took up that role, with The Doctor serving as their Scientific Advisor for a time, although he never officially resigned.
Even with The Doctor helping them on occasion, they are the primary and official face for the defense of Earth in the Doctor Who universe.
This logo is a remake of the original logo of UNIT first seen during Patrick Troughton’s time as the Doctor, and used at various times throughout the classic series.