A warlord was an individual who exercised control over an area through armed force.
In 2098, Vulcan master Sklar stated that Surak had brought down the fiercest warlords Vulcan had ever seen with his words. (ENT novel: Surak's Soul)
M'k'n'zy of Calhoun was warlord of Xenex during the Xenexian uprising against the Danteri. (NF novel: Martyr)
In the 24th century, the Alrakis system was controlled by several warlords. (DS9 - Prophecy and Change short story: "... Loved I Not Honor More")