Definition from Wiktionary, the free dictionary
From Japanese 横綱 ‘wide rope’, the symbol of their rank worn about the waist.
yokozuna (plural yokozuna)
- (sumo) The highest rank of sumo wrestler, above ozeki; a grand champion.
- 1997, A yokozuna is the very highest rank in sumo – “just like Mameha's position in Gion,” as Nobu explained it to me. I had no reason to doubt him; but if Mameha ever took half as much time entering a party as these yokozuna took entering the ring, she'd certainly never be invited back. — Arthur Golden, Memoirs of a Geisha (Vintage 1998, p. 202)
Usage notes 
- Technically, yokozuna is an honorary title, rather than a rank.