The definition of a diva

The Ahmanson Theatre is housing a divinity in human guise, one capable of generating audience frenzy akin to accounts of Judy Garland’s closing at the Palace or Maria Callas’ return to the Met. The goddess in question is the luminous Barbara Cook, whose “Mostly Sondheim” concludes its limited engagement on Sunday. This 2002 Tony- and Olivier-nominated celebration of Stephen Sondheim and the songs he covets finds Broadway and cabaret singer Cook in astonishing form.

At an age when mere mortals are perusing their clippings and popping Geritol, Cook remains a force of musical nature. This is everywhere apparent -- in her commitment to interpretive truth, her searching intelligence and certainly her joyous soprano, one of the most celebrated in Broadway history. If the decades since her 1951 “Flahooley” debut have inevitably left their traces -- more caution on high, breathier legato, darkened timbre -- they are only traces, irrelevant against the eloquent refulgence Cook still commands.

“Mostly Sondheim” starts casually, with the black-clad chanteuse strolling on, her sunbeam face illuminating the space in tandem with D. Martyn Bookwalter’s superb lighting. In perfect attunement with musical director Tex Arnold (piano) and Dave Carpenter (bass), Cook launches a jocular yet taut “Everybody Says Don’t” from Sondheim’s “Anyone Can Whistle.” Pandemonium ensues, and she’s just beginning.

Cook charts her airtight program in delectable conversations with her audience. She conceived this show in response to the composer and lyricist’s 70th birthday list of songs he wishes he’d written. Weaving selections from that imposing array with choice examples of Sondheim’s own art, Cook makes a convincing case for Sondheim’s supremacy, stressing his integrity and demonstrating her own in the process.

Harold Arlen and E.Y. “Yip” Harburg’s little-known “Buds Won’t Bud” rocks, with Cook selling Harburg’s droll lyrics with impeccable articulation. This quality also serves Sondheim’s “You Could Drive a Person Crazy” from “Company,” in her hands an ironic tour-de-force.

Humor is one constant in Cook’s lexicon; generosity of spirit is another. Her advocacy of Sondheim’s taste deftly encompasses Reba McEntire’s Broadway debut in the “Annie Get Your Gun” revival. In tribute, Cook offers a priceless “You Can’t Get a Man With a Gun” and a scorching “I Got Lost in His Arms.” Vaudevillian drive marks Cook’s medley of Sondheim’s more unexpected faves, beginning with “Hard Hearted Hannah.” Although Cook hails from Atlanta rather than Savannah, her delivery is Georgia-peachy, regardless. Segueing into “Waiting for the Robert E. Lee,” then “San Francisco,” she blossoms into every Orpheum Circuit headliner who ever stopped a show.

Cook’s exquisite reading of Arlen and Johnny Mercer’s “I Had Myself a True Love” justifies its art song description. Arlen and Harburg’s “The Eagle and Me,” from “Bloomer Girl,” finds her moving the socially aware material from her solar plexus to that of the listener.

Her gorgeous “Happiness,” from Sondheim’s “Passion,” traces an inward spiral worthy of Poulenc, and “Loving You” from the same show is unbearably moving. So are the two “Follies” numbers that Cook now owns, “In Buddy’s Eyes” and “Losing My Mind.” And her “Send In the Clowns,” accurately locating that standard’s core of angry hurt, is revelatory. .

Finally, there are two representative, late-inning benchmarks of Cook’s heavenly art. The evening’s pinnacle is Jerry Bock and Sheldon Harnick’s “Ice Cream,” from “She Loves Me,” which Cook introduced in 1963. Her 2003 version is largely identical, and spot-on. At the opening performance, however, Cook halted in mid-descent from the climactic high B, stating, “I can do that better.” And she did, repeating the ending and bringing down the house.

The other touchstone is her encore, which scores a heart-stopping coup. Abandoning the microphone, Cook imbues “Anyone Can Whistle” with mystical simplicity, her unassisted purity reaching the top balcony of the spellbound theater. Such magic is the stuff that creates legends, and designates Cook as a national treasure.

“Mostly Sondheim” is entirely sublime; to miss it would constitute sacrilege.

*

‘Mostly Sondheim’

Where: Ahmanson Theatre, 135 N. Grand Ave., L.A.

When: Thursday-Saturday, 8 p.m.; Sunday, 2 p.m.

Ends: Sunday

Price: $20-$55

Contact: (213) 628-2772

Running time: 1 hour, 35 minutes