NACHA. It's a nickname she has been called since childhood; it doesn't really mean anything. Her given name is Clothilde. Guevara is not her real name either; she made it up, choosing the first few letters of her stepfather's name. She thinks now that she was subconsciously trying to reach a rapprochement with him. Like everything else about her, the source of her name is complicated, and original.

Nacha Guevara is an Argentine who is attempting to carve out for herself a special kind of theatrical entertainment, which can be seen for two weeks, starting Tuesday, at the Kennedy Center's Terrace Theater. She sings, she dances, she performs.

"She is like no one else I've ever seen," said producer/director Harold Prince, who discovered her in Spain and launched her here with a spectacular one-night Broadway concert four years ago. "She is a chameleon . . . she has Dietrich-like looks, but more eccentric. She's like Jacques Brel in the individuality of her material and the individuality of her performance. She does comedy stuff, and glamor stuff; she's into theatrics. And when she does some political poems by Jose Marti, I'm just overwhelmed."

Politics is something Guevara has not shied away from: It was the reason she left Argentina, despite her increasing popularity. Reviewers have said she is like Piaf, like Garland, but without their sentimental vulnerability. "I am an actress who learned to sing," she says.

She is wearing a dark green dress designed by Norma Kamali, who makes couture clothes out of sweatshirt material. It has a high collar and tight cuffs, and a shawl of bright red, which matches her perfectly made up lips. Her gold hair is tucked up in a bun, emphasizing the angularity of her face and wide brown eyes. She has a dancer's body, of birdlike spareness, and an innate elegance that has undoubtedly helped her travel from an unmoneyed childhood to the cosmopolitan boulevards of the world's capitals. She is 42, but looks at most a sophisticated 32.

Her show will be 70 percent in English and 30 percent in Spanish, and will draw on such composers as Stephen Sondheim, Frank Loesser, Noel Coward and Jacques Brel. Aside from an outsized tuxedo (she used to wear a fitted one but dropped it when it became more popular), her costumes have been designed by William Ivey Long, who won a Tony for his clothes in "Nine." The set is by Duke Durfee, who did "Joseph and the Amazing Technicolor Dreamcoat." The musical arrangements and accompaniment are by Alberto Favero, who is also her third husband.

"Every human experience in life is the same," she says in Spanish-flavored English. "Death, life, craziness, illusion, power, loneliness. They are present in every single human expression. We address those things.

"What I do is what I like. It is an exercise of expressive freedom."

She and Favero, who have been together 14 years, left Argentina in 1974, fleeing like so many others from a country in which leftists "disappeared" regularly. Protest songs, many of which poked fun at the military, had been among her standard repertory. One Argentine expert described her as the Joan Baez of Argentina.

In 1975 Guevara returned home at the behest of her impresario; on opening night, she says, a bomb went off in the theater and killed two people. She and Favero left, and she has not gone back since.

"We don't belong to any political party and never will," she says firmly. "But we will continue working for human rights."

For four years after leaving Argentina, they traveled through South America and Spain, performing. Then, like the proverbial Good Fairy, Harold Prince heard her recording, in Spanish, of "Don't Cry for Me, Argentina," and invited her to audition for the lead role in "Evita" in London.

"We flew from Madrid for the audition. It was a magic moment. My first audition."

She sang five songs. Then "he asked to go on stage to kiss me. The whole cast was there. Everyone embrace, we cry. It was magic."

"She showed up to audition in London with an entourage," recalled Prince. "That turned me off--I thought who is this person I've never heard of acting like a star? Then after the audition I said, 'Evita' is all wrong for you, it's a waste. You have 50 things going on, and it wouldn't draw enough on them."

She wasn't right for the part, and it wasn't right for her. She doesn't like the cynical interpretation of Eva Peron the show conveys, nor the lyrics of the songs. "She was a star, really a star," Guevara has said. "I'm not a Peronista . . . but she was a true woman and nobody can deny that. She was very generous with her own life. She gave her life, really."

The audition was not a waste, however. "Hal Prince knows more about musical theater than anyone in the world," she says, gesturing with elegant fingers. "I went to London for the opportunity of meeting Hal. There began a friendship, an extraordinary friendship. He has been so generous." And even though she did not play Evita, she will wear the dress from the original production in her show.

At any rate, Prince and his wife, Judy, heard her a few months later in Majorca, Spain, where he has a home. Plans were hatched for the Princes to present her to an invited audience of 1,000 at New York's St. James Theatre, which was the kind of night that "a part of every actress dreams about." "My wife invited every known living media person in New York," said Prince. The audience was filled with beautiful people that Women's Wear Daily was on hand to chronicle, and chic dinner parties were held after the show. Guevara even got a standing ovation in Sardi's. "Judy said it was like a bad movie," Guevara joked.

Vogue photographed her and Favero, and they were what "People are talking about . . ." But the offers did not exactly flow in. The show was almost entirely in Spanish, and that was a problem, she thinks. She got a date in Chicago, and once again the critics and audiences were enthusiastic, but then she went back to touring in Mexico, Brazil and Spain.

"It the St. James concert was a magic moment. While I was preparing I tried to think only that I should enjoy that night . . . But I wasn't prepared for the night after. For the silence. It was like a little depression after that; it was natural."

She went back to New York a year later for a run at the Manhattan Theatre Club; again the raves came. "They were sold out, but after the review came out in The New York Times, they had a waiting list!" says Favero. "I have never heard of such a thing in a theater."

But even though she was a hit, the show could not be extended because of previous scheduling. There was talk of another show, but that fell through. Gradually Guevara and Favero came to the decision that they had to do a new show, mostly in English, and it is this show they have been rehearsing six hours a day in New York, choreographing every detail, in preparation for Wednesday night's opening. Prince will come to see Tuesday's preview.

They have settled into a Manhattan brownstone, put the 15-year-old and the 10-year-old in school (her 20-year-old son has returned to Argentina and his father), and decided to stay a while. They want to make it here, and they say it is easier for "healthologists and naturists" like themselves to manage in the health-conscious U.S.A.

They are both vegetarians, and three years ago gave up white sugar and flour. Before that she loved pizza.

Guevara is a late bloomer, who did not discover her metier until her late twenties. The daughter of a mulatto soccer player she never knew and a mother who wanted to be a singer, she went to dancing school all through her childhood, and it was assumed that she would be a dancer.

"But I rebelled. I stopped dancing. For two years I had nowhere to go. I get married; I have my child. Twenty to 25 was a very difficult moment for me."

Then she returned to dancing, and also became a fashion model, which she did not find much of a challenge. "So I decided to become an actress." She took lessons, and landed parts in musicals and plays like "A Delicate Balance" and "Hamlet."

"But something was missing. I knew something else was inside, but I didn't know the way to put it out."

The next "magic moment" occurred when she put together a show of 12 songs for what would be called a showcase in this country. "When I finished the thing--that's all I could call it, I did not know what it was--I said that's what I want to do. For the first time in my life I feel free, open and able to communicate."

"I think she can be immensely popular," said Prince. "Like Bette Midler. Except she's not like Bette Midler. She's one of a kind."