From Wiktionary, the free dictionary

• IPA^(key): /ˈɹeɪnɪŋ/
• Rhymes: -eɪnɪŋ
• Homophones: raining, reining

reigning (not comparable)

1. Currently ruling or holding a position.

   The reigning world heavyweight champion said he'd defend his title once more and then retire even if he won.

   • 1945 July and August, “Notes and News: Duplicate Locomotive Names”, in Railway Magazine, page 235:

     The reigning monarch, King George VI, is commemorated by two express engines—G.W.R. "King" 4-6-0 No. 6028 and L.M.S.R. streamlined Pacific No. 6244— […] .

reigning

1. present participle and gerund of reign