Sideburns were a variant of facial hair; they were thick patches of whiskers on the sides of one's head.
The Third and Fourth Doctor both wore sideburns (TV: Spearhead from Space [+]Loading...["Spearhead from Space (TV story)"], TV: Planet of the Spiders [+]Loading...["Planet of the Spiders (TV story)"]) - the Fourth's were red even though the hair on his head was dark brown. (PROSE: The Drosten's Curse [+]Loading...["The Drosten's Curse (novel)"]) The Tenth Doctor described the Fourth Doctor as being all "scarf and sideburns". (AUDIO: Out of Time [+]Loading...["Out of Time (audio story)"])
The Tenth Doctor was delighted to see he had grown sideburns after his regeneration. (TV: Born Again [+]Loading...["Born Again (TV story)"]) He kept them to the end of his life, (TV: The End of Time [+]Loading...["The End of Time (TV story)"]) though he was once temporarily aged to a point he lost them, and later, all of his hair. (TV: The Sound of Drums [+]Loading...["The Sound of Drums (TV story)"], Last of the Time Lords [+]Loading...["Last of the Time Lords (TV story)"]) His fourteenth self, who was almost identical to his tenth, also had sideburns. (TV: The Power of the Doctor [+]Loading...["The Power of the Doctor (TV story)"], COMIC: Liberation of the Daleks [+]Loading...["Liberation of the Daleks (comic story)"])