A warlord is a military leader who holds real power over an area by force, without a government giving him the right to. He has his own soldiers, who are loyal to him and not to a country. He may collect his own taxes, control roads, and run the place as if it were his. You see the word in news about countries where the central government has become weak — the land splits, and each piece falls to whoever has guns. It is not a job title and not a rank; nobody is appointed warlord. It is what other people call him. The word carries judgement: it says this power is not proper, however real it is. Historians also use it for the past — Chinese history from 1916 to 1928 is called the Warlord Era in English, and Japan's old regional lords are sometimes called warlords in English books. It is a countable noun: a warlord, local warlords. You may also meet it stretched for fun — a business writer might call a fierce boss a warlord — but that is a joke that leans on the fear in the real word.