One reason people use the word "diva" too often nowadays is that the notion of great women being defined by great performances has made great fodder for modern dramatists. Not far beneath Billy Wilder's Sunset Boulevard, the pinnacle of diva dramaturgy, is Terrence McNally's Tony Award-winning 1996 play, Master Class. The subject here, in a command performance by Rita Moreno, is the aching soul of superstar Maria Callas. But the show is also a valentine to the opera, beautiful bitch that she is. McNally knows how to orchestrate for human instruments. His play is agreeably operatic, and director Moisés Kaufman planes the lines of its shapely form with affection. Based on classes Callas taught at Juilliard in the early '70s, the action is an imparting of her earned wisdom. She's that dazzling -- the brutal teacher we've all had or wanted. Among the three students, strong singers all, Sherry Boone's Sharon is the real crowd-pleaser; the arc of her creative process is the most impressive and the most human. Mark Wedland's design and David Lander's lighting make good use of a deep, high-ceilinged stage to reveal a few stirring glimpses of grandeur. And Moreno, firmly rooted and lifting her face to gather the light, makes a big room feel small. "'The real world.' Brutal expression, brutal place," she declares as Callas, edifying performers everywhere.