A stubble, also called a 5 O'Clock Shadow, was made up of stiff hairs growing on a part of the body that has not been shaved for a while, especially on a man’s face.
The War Doctor had a stubble in his younger days; most notably when he and Petrella escaped a Mark VIII Dalek ambush led by a Type E Supreme Dalek. (COMIC: Ambush [+])
The Ninth Doctor had a stubble; (TV: Rose) which he occasionally shaved away. (TV: The Empty Child)
The Fifteenth Doctor had much of his facial hair forming a stubble. (TV: The Power of the Doctor-The Giggle)
The Spy Master had a stubble when posing as 'O' to set the Thirteenth Doctor up. (TV: Spyfall)