A leader was someone who in charge of others.
Ergu was the leader of the Tarls when the Fourth Doctor and Leela visited E9874. (AUDIO: The Exxilons)
The Authority was the leader of the Vrill. (AUDIO: Survival of the Fittest)
Mike Leader was a fictional character created from the mind of a young girl on the Crooked World. He was the leader of the Skeleton Crew. (PROSE: The Crooked World)
Fadros Pallujikaa was the leader of the Sycorax who tried to take over the Earth in 2006. (COMIC: The Widow's Curse, TV: The Christmas Invasion)