Wordnet 3.0
NOUN (1)
1. 
 a ruler of the eastern Roman Empire; 
The Collaborative International Dictionary of English v.0.48:
basileus \basileus\ n.
   a ruler of the eastern Roman Empire.
   [WordNet 1.5]
WordNet (r) 3.0 (2006):
basileus
    n 1: a ruler of the eastern Roman Empire