The Collaborative International Dictionary of English v.0.48:

Reign \Reign\ (r?n), v. i. [imp. & p. p. Reigned (r?nd); p.
   pr. & vb. n. Reigning.] [OE. regnen, reinen, OF. regner, F.
   r['e]gner, fr. L. regnare, fr. regnum. See Reign, n.]
   1. To possess or exercise sovereign power or authority; to
      exercise government, as a king or emperor;; to hold
      supreme power; to rule. --Chaucer.
      [1913 Webster]

            We will not have this man to reign over us. --Luke
                                                  xix. 14.
      [1913 Webster]

            Shall Banquo's issue ever
            Reign in this kingdom?                --Shak.
      [1913 Webster]

   2. Hence, to be predominant; to prevail. "Pestilent diseases
      which commonly reign in summer." --Bacon.
      [1913 Webster]

   3. To have superior or uncontrolled dominion; to rule.
      [1913 Webster]

            Let not sin therefore reign in your mortal body.
                                                  --Rom. vi. 12.
      [1913 Webster]

   Syn: To rule; govern; direct; control; prevail.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "reigning":
   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,
   ordinary, pandemic, paramount, popular, predominant, predominate,
   predominating, prepollent, preponderant, preponderate, prepotent,
   prevailing, prevalent, rampant, regnant, regulating, regulative,
   regulatory, rife, routine, ruling, running, sovereign, standard,
   stereotyped, supreme, usual