The term "jazz diva" gets tossed around fairly indiscriminately; it's been used to describe just about every improvising vocalist at one time or another. But thanks to her coloratura technique--replete with a startling dramaticism, flawless intonation, and a vibrato that can get as wide as the Mississippi--Morgana King may actually deserve the honorific. Disagreement might come more readily from the jazz camp than from the opera lovers. King's bell-like soprano produces notes of an ethereal purity, like the supernatural tones you get by rubbing the rim of a glass; to get them, King makes frequent use of her "head voice," a technique taught classical singers for the relaxed manipulation of the highest registers. (It's an operatic soprano's best friend.) And while she concentrates on the jazz repertoire, King doesn't scat-sing or improvise in any other traditional manner; her jazz comes instead from the balletic actions--the swoops, leaps, and graceful dives--of her unmistakable voice. As with all singers, King's voice has naturally darkened over the years. This, along with the expected exaggeration of her stylistic hallmarks, has only made her performances more theatrical, turning virtually every song into a musical scenelet (as befits the woman who played Don Corleone's wife in The Godfafher). Friday and Saturday, 9 and 11 PM, Cotton Club, 1710 S. Michigan; 341-9787.