WHAT exactly is a portrait?

When it established the National Portrait Gallery in 1962, Congress responded to that question with the most traditional of answers. A portrait, said the lawmakers, is "a painted or sculpted likeness." For years that dreary definition stunted the museum. When the Gallery displayed its formal portraits, its marble busts and paintings of somber-suited gentlemen posed in throne-like chairs, it was not merely being staid. It was following the law.

The law was amended last October, and the Gallery has since been liberated. It is still acquiring "portraits," but by establishing a new department of photographs, it has exploded the old meaning of that term. For photography leads to film, film clips lead to video and video suggests new electronic art forms that have not yet been invented. A door has been thrown open.

The photos of President Van Buren, J.P. Morgan, Leadbelly and others that have just gone on display there are not in themselves radical. But they imply a revolution in the traditional relationship between museum curators and the portraits they collect.

Consider, for example, the issues raised by numbers, by the proliferation of portraits made with film. Mathew Brady's daguerreotype of Van Buren fills a gap in the museum. (It is the only portrait of the President in the Gallery's collection.) The picture is unique, since daguerreotypes, like marble busts, are one-of-a-kind objects. And because Brady made it, it also has importance as a work of art.

Such old artifacts are rare, but countless photos of the famous are available today.

Imagine what will happen when Marvin Sadik, the director, and William Stapp, his curator of photographs, or their successors, begin selecting portraits of, say, Richard Nixon.

President Nixon must have stepped before one camera or another 100,000 times. His likeness has been captured on many thousands of hours of video tape and film. The same, of course, applies to Johnny Carson, Muhammad Ali and other figures of our time.

How will curators of photographs begin selecting "portraits" of such men? Will the Gallery collect casettes of "The Tonight Show," the "Thrilla in Manilla" and the "Checkers" speech? Or will its curators attempt to screen the countless photos of these men in the bulging archives of the government, the newspapers and magazines?

One thing is quite certain. No one can select, say, 10 photographs of Nixon, display them in the Gallery, and say, "There, that's what he looked like, that is who he was."

The curator who does so is no longer just the curator. He has become the portraitist. Less curator than artist, he is as responsible as the photographers for the portraits on display.

Portraits can be beautiful, but accuracy is more important. Because portraitists should show us what their subjects looked like, the camera serves them well. And not just the still camera.

When the Gallery begins collecting video tape and film, sticky curatorial issues are certain to arise. If a portrait is no longer just a painting or a sculpture, what then are its boundaries? Are there any limitations?

Should a portrait move? Should the Gallery acquire films or video tapes of a dancing Fred Astaire, a strolling Harry Truman, or John Havlicek stealing the ball? Or should it instead search out static likenesses it can hang upon its walls?

Should the Gallery acquire only portraits that are silent, or might some of them make noise? A clip of Groucho Marx talking fast and doing something utterly preposterous is arguably a better likeness than a painting of his face. Once sound is admitted, need the face be there at all? Should the Gallery collect recordings of Ray Charles and Martin Luther King?

Should a portrait be unique?Most museums would say yes, and most paintings, busts and drawings are; but once the door is opened to portraits made by camera, uniqueness matters less, and eventually not at all. A record of the Beatles made with sound effects and multiple-track recordings is not a reproduction of songs performed in concert. It is an original. The same applies to video. Should the museum buy casettes that can be endlessly reproduced?

Should the Gallery confine exhibits to its halls? Or should it transmit taped television portraits to a million private homes?

Things were simpler by far when the Gallery was forced, by law, to confine its acquisitions to "painted or sculpted likenesses." The reasons for imposing that severe restriction were political, not esthetic. The painting-and-sculpture-only rule was written into law in 1962 at the request of L. Quincy Mumford, then Librarian of Congress. The library, in those days, held something of a corner on the U.S. government's photography collections. Mumford did not want the neophyte National Portrait Gallery trampling on his turf.

The law was on its face petty and anachronistic. It was also antidemocratic. The Gallery, after all, was established to pay homage to those "men and women who have made significant contributions to the history, development and culture of the people of the United States." Under the restrictive clause, many people, though "significant," would have been excluded. Only those who had the time and money or, perhaps, the vanity, to sit for painters or sculptors would have been admitted to the museum.

Within weeks of his appointment as Gallery director, Sadik began lobbying the Congress for a change. Thoreau, he would point out, was missing from the Gallery because daguerreotypes were excluded. The same clause had banned Van Buren from the Hall of Presidents. Sadik pressed his point. With the aid of Rep. Lucien N. Nedzi (D-Mich.) he eventually prevailed.

Quietly last October, the National Portrait Gallery Act was amended as follows: "The term 'portraiture' includes portraits and reproductions thereof made by any means of process, whether invented or developed heretofore or hereafter."

The Gallery's new photograph collection is the first fruit of that change.

So far William Stapp, the new curator of photographs, has been able to combine in his small collection both biography and art. That 1903 photograph of J.P. Morgan is the work of Edward Steichen, that portrait of James Agee is by Walker Evans, that rather hokie Leadbelly is by Berenice Abbott, and that informal shot of Franz Kline, the painter, is by an artist as important, the photographer Robert Frank.

These photographs and others of Frank Lloyd Wright, Daniel Webster, Isadora Duncan, Mark Twain and Henry James are now on display, but not all the photographs the Gallery has acquired are by famous artists. It owns impressive portraits of Frederick Douglass, Woodrow Wilson, Teddy Roosevelt and Horace Greely that were taken by photographers whose names are still unknown.

Stapp says the Gallery is spending more on photographs that any other Washington Museum. The Smithsnian Institution gives the Gallery approximately $250,000 annually for new acquisitions. One-fifth of that amount is being used to purchase photographs.