The Collaborative International Dictionary of English v.0.48:

Abbot \Ab"bot\ ([a^]b"b[u^]t), n. [AS. abbod, abbad, L. abbas,
   abbatis, Gr. 'abba^s, fr. Syriac abb[=a] father. Cf. Abba,
   Abb['e].]
   1. The superior or head of an abbey.
      [1913 Webster]

   2. One of a class of bishops whose sees were formerly abbeys.
      --Encyc. Brit.
      [1913 Webster]

   Abbot of the people. a title formerly given to one of the
      chief magistrates in Genoa.

   Abbot of Misrule (or Lord of Misrule), in medi[ae]val
      times, the master of revels, as at Christmas; in Scotland
      called the Abbot of Unreason. --Encyc. Brit.
      [1913 Webster]

WordNet (r) 3.0 (2006):

abbot
    n 1: the superior of an abbey of monks [syn: abbot,
         archimandrite]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "abbot":
   abbacomes, ascetic, beadsman, brother, caloyer, celibate, cenobite,
   conventual, conventual prior, friar, grand prior, hermit,
   hieromonach, lay abbot, lay brother, mendicant, monastic, monk,
   palmer, pilgrim, pillar saint, pillarist, prior, religieux,
   religious, stylite