The Collaborative International Dictionary of English v.0.48:

Preponderant \Pre*pon"der*ant\, a. [L. praeponderans, -antis:
   cf. F. pr['e]pond['e]rant. See Preponderate.]
   Preponderating; outweighing; overbalancing; -- used literally
   and figuratively; as, a preponderant weight; of preponderant
   importance. -- Pre*pon"der*ant*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

preponderant
    adj 1: having superior power and influence; "the predominant
           mood among policy-makers is optimism" [syn: overriding,
           paramount, predominant, predominate,
           preponderant, preponderating]