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]

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