Valeyard

Valeyard was a title which the Sixth Doctor knew to mean "learned court prosecutor". (TV: The Mysterious Planet) In the Sixth Doctor's trial, the title was claimed by a Time Lord known as the Valeyard, who revealed to be an amalgamation of the Doctor's darker sides. (TV: The Ultimate Foe)

Behind the scenes
Contrary to the in-universe definition, "Valeyard" is not a genuine legal title meaning "learned court prosecutor." The word was entirely made-up by writer Robert Holmes. The title does bear some similarity to the French word "vieillard", meaning "old man".