When Lena Horne strides onstage at the Warner Theater, dressed in flowing white jersey and sparkling like the diamonds on her finger, you might get a momentary flash of that advertisement: what becomes a legend most? Or rather, what is a legend, and why?

During the ensuing two hours she shows that, in her case at least, a legend is someone who does something no one else can, in a way no one can imitate. She combines elegance and funkiness, beauty and brass, musicianship and showmanship and, in short, knocks everyone's socks off.

Lena Horne has set a new standard for the theatrical category called "one-woman show." In the first place, she has the sense to use a 16-man orchestra, led by an adept woman conductor, as well as a trio of excellent young dancer-singers, so it is not, strictly speaking, one-woman.

The important element, however, is her, and what she does is not nightclub, cabaret, or variety show, but a combination of songs, anecdotes, commentary and--as she notes several times--sweat. Through it all, the key to its success is her own sense of self, the way she pokes fun at her elegant image and her sultry screen past, her revelation that she can see the audience and watches what they're doing, wry references to her age (65, but it seems a meaningless number), her occasional ribaldry. What she produces is a sort of intimate spectacular, with Broadway dazzle and living room camaraderie.

"This is my Hollywood walk," she said at one point, demonstrating a sophisticated glide. Then, "here's mine:" a sort of loping boogie. She sings "I Want To Be Happy" in several personalities, from a coy child to a snob to a red-hot mama. She is playful, flirtatious and not above a few bumps and grinds, not to mention a merciless series of digs at producer James M. Nederlander. The classic "Stormy Weather" gets two treatments, the first the standard version. Near the end of the show, she says she didn't know how to sing it 40 years ago when she first sang it, and gives it a pull-out-the-stops rendition, the way she said she heard Ethel Waters do it.

Above all, she is in control. Every gesture, every note and every ad lib is the work of a virtuoso, an athlete who knows each muscle. When a heckler moaned something unintelligible, she said, "Somebody help my friend there," and carried on. She announces that when she is reincarnated she wants to spend "10 minutes as Tina Turner," and then proves that she really has nothing to wait for, boogying to beat the band.

If there is a theme in the show it is a subtle one, the message that life is to be lived to the hilt despite the rough parts that come along the way. But there is also the message of Lena Horne, who sings and personifies the values of a strong, independent womanhood without making a point of it, who shows that theatricality at its best is magic, and that age really has nothing to do with life.

"Lena Horne: The Lady and Her Music," concept by Horne and Sherman Sneed, produced by James M. Nederlander, Michael Frazier, Fred Walker and Sneed, music conducted by Linda Twine, directed by Harold Wheeler, scenery by David Gropman, costumes by Stanley Simmons, her gowns by Giorgio Sant'Angelo, lighting by Thomas Skelton, with Clare Bathe', Marva Hicks, L. Edmond Wesley.

At the Warner Theater through May 29.