Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. Having the chief power; ruling; predominant; prevalent.
      ``A traitor to the vices regnant.'' --Swift.

Source : WordNet®

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