For Nadia Comaneci, the notion is unspeakable.

For how much longer, she is asked, does she plan to continue in competitive gymnastics?

"I am never," she said, her dark eyes flashing, "even thinking about something like that."

At 19 Comaneci, who in 1976 dominated women's gymnastics at the Montreal Olympics in a manner that few athletes have dominated a sport, is the acknowledged "old lady" of the Romanian women's gymnastics team, which finished a 3 1/2-week American tour yesterday at Capital Centre.

She is a head taller than the other members of her gymnastics team and more poised and mature. In Montreal, at 14, she reached the pinnacle of world gymnastics, earning an unprecedented seven scores of 10 and winning three gold, two silver and one bronze medals.

In those five years the word Nadia has become a household word, synonymous with gymnastic excellence, youth and beauty. "Nadia's Theme," the music she performed to, sold a million records and its lilting melody was adopted as the theme song of a television soap opera. She was on the cover of Time, Newsweek and Sports Illustrated. The extensive media coverage triggered an unprecedented surge in youth gymnastics throughout America.

She won two gold medals at the Moscow Olympics last summer, but failed to dominate as she had done four years earlier. She lost to Soviet gymnast Yelena Davydova for best all-around gymnast in what is still a heatedly debated decision.

"Nadia was shafted in Moscow," said Rick Appleman, the Greenwich, Conn., sports promoter, who organized the Nadia 81 gymnastics road show. "There are many, many people who still think she's the best gymnast in the world."

It also is a fact that, at 19, Comaneci has passed the point where careers of most women gymnasts have peaked, and it is unlikely she will ever repeat anything like her Montreal triumph.

As she says, she does not think about the time when she will no longer compete, but she is making plans for the future.

Asked what comes after Nadia 81, she said, "Nadia 82, of course," but at the same time she is planningg a career in coaching. She is a college freshman, studying at the University of Bucharest's Institute of Physical Education.

In part because there was little television coverage of the Moscow Olympics, it was difficult for another gymnast to dethrone Comaneci in the hearts and minds of the American public in 1980, as Comaneci in 1976 had done to Olga Korbut, standout Soviet gymnast at the 1972 Munich Olympics.

Nowhere could this have been clearer yesterday that at Capital Centre just before intermission. As the strains of "Nadia's Theme" were piped throughout the vast arena, Comaneci walked along to the middle of the floor as an announcer introduced her as "a symbol belonging to the whole world."

Then Kurt Thomas, the American gymnast co-starring in the show, appeared from the sideline, handed Comaneci a bouquet of fresh flowers, kissed her on the lips, and the couple walked off hand in hand. The crowd loved it, and a sea of flashbulbs exploded as hundreds of fans recorded the moment for posterity.

But in fact, Comaneci's participation in Nadia 81 was held to a minimum. Members of the Romanian national team and the American men gymnasts did the bulk of the vaulting, tumbling and balance beam and bar exercises.

Often described as aloof and reticent, Comaneci showed no hesitancy during a meeting with members of the media at a practice session at the Marvateens gymnastics center in Rockville.

"No. I am not shy. I can smile," said Comaneci, who powdered her nose before a television appearance.

An interpreter was present, but Comaneci did not need him, answering questions in slightly accented English. Next to gymnastics, she said, she likes disco music, and she still collects dolls. She has 300 of them.

She likes gymnastic tours of America because she likes promoting Romanian gymnastics and, according to promoter Appleman, she is performing better and enjoying it more than she did on a previous tour in 1977.

"I created the tour exhibitions with Olga Korbut," Appleman said. "When Nadia happened, I inherited her."