Logo
A logo was a symbol or emblem used to promote a brand.
PhiCorp used the letter Phi as its logo. (TV: The Gathering)
The Fifth Doctor recalled that his book series was to be named with a comma, The Doctor, Who Discovers but became Doctor Who Discovers after "some idiot" in the publisher's design department missed the "The", creating a logo that said Doctor Who. (AUDIO: The Kingmaker)
Other realities
Cybus Industries used the letter C as its logo. This logo appeared on most of its products, such as the Cybus EarPods. The Cybermen created by Cybus Industries also had the logo prominently displayed on their chests, which prompted the Tenth Doctor to remark that John Lumic was turning them "into a brand". (TV: The Age of Steel [+]Loading...["The Age of Steel (TV story)"] etc.)
Behind the scenes
Information from invalid sources
The TARDIS once flew through the Doctor Who logo, leaving a line behind it, striking through the logo. (NOTVALID: The NEW Doctor Who Logo)