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Basileus was the title given to Seleucus, one of Alexander the Great's generals. After Alexander's death, Seleucus established the Seleucid Empire, which ruled over a vast territory in the Middle East. The title "Basileus" was commonly used by Greek rulers and meant "king" or "sovereign." It reflected Seleucus's position as the supreme ruler of his empire and his authority over its people and territories.