The Collaborative International Dictionary of English v.0.48:

Chieftain \Chief"tain\, n. [OE. cheftayn, chevetayn, OF.
   chevetain, F. capitaine, LL. capitanus, fr. L. caput head.
   Cf. Captain, and see chief.]
   A captain, leader, or commander; a chief; the head of a
   troop, army, or clan.

   Syn: Chief; commander; leader; head. See Chief.
        [1913 Webster] Chieftaincy

WordNet (r) 3.0 (2006):

chieftain
    n 1: the leader of a group of people; "a captain of industry"
         [syn: captain, chieftain]
    2: the head of a tribe or clan [syn: headman, tribal chief,
       chieftain, chief]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "chieftain":
   anointed king, boss, chief, cock, crowned head, dynast, emperor,
   grand duke, head, headman, hierarch, high chief, honcho, imperator,
   king, king-emperor, kinglet, leader, lord paramount, majesty,
   master, monarch, overlord, paramount, petty king, potentate,
   prince, prince consort, royal, royal personage, royalty, ruler,
   sovereign, suzerain, tetrarch