The Collaborative International Dictionary of English v.0.48:

Overlord \O"ver*lord`\, n.
   One who is lord over another or others; a superior lord; a
   master. --Freeman.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overlord
    n 1: a person who has general authority over others [syn:
         overlord, master, lord]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "overlord":
   anointed king, boss, bwana, chef, chief, chieftain,
   church dignitary, crowned head, dynast, ecclesiarch, elder,
   emperor, employer, goodman, grand duke, guru, high chief, husband,
   imperator, king, king-emperor, kinglet, liege, liege lord, lord,
   lord paramount, majesty, master, monarch, padrone, paramount,
   paterfamilias, patriarch, patron, petty king, potentate, prince,
   prince consort, rabbi, royal, royal personage, royalty, ruler,
   sahib, seigneur, seignior, sovereign, starets, suzerain, teacher,
   tetrarch