The Collaborative International Dictionary of English v.0.48:

Regnant \Reg"nant\ (-nant), a. [L. regnans, -antis, p. pr. of
   regnare to reign: cf. F r['e]gnant. See Reign.]
   1. Exercising regal authority; reigning; as, a queen regnant.
      [1913 Webster]

   2. Having the chief power; ruling; predominant; prevalent. "A
      traitor to the vices regnant." --Swift.
      [1913 Webster]

WordNet (r) 3.0 (2006):

regnant
    adj 1: exercising power or authority [syn: regnant,
           reigning, ruling]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "regnant":
   ascendant, at the head, average, besetting, boss, chief,
   commanding, common, controlling, current, dominant, epidemic,
   general, governing, head, hegemonic, hegemonistic, in ascendancy,
   in charge, in chief, in the ascendant, leading, master, normal,
   on the throne, ordinary, overbearing, pandemic, paramount, popular,
   predominant, predominate, predominating, prepollent, preponderant,
   preponderate, prepotent, prevailing, prevalent, rampant,
   regulating, regulative, regulatory, reigning, rife, routine,
   ruling, running, sovereign, standard, stereotyped, supreme,
   swaying, usual, widespread



Bouvier's Law Dictionary, Revised 6th Ed (1856):

REGNANT. One having authority as a king; one in the exercise of royal
authority.