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]