Claude Monet, well-represented in the new impressionism show at the St. Louis Art Museum, was probably thinking only positive thoughts when he used a variant of the word in the title of a canvas he produced in 1872. But two years later, when the painting was exhibited in Paris, a reviewer appropriated the word and twisted it into a barb. Monet called his work ''Impression: Sunrise.'' The critic Louis Leroy in turn wrote that Monet was a mere ''impressionist,'' an artist who - possibly shallow-minded, possibly ill-equipped with technique - abandoned the traditional painterly values of detail and clarity for the sake of self-indulgent blurs.

More than a decade later, a comparable reprimand introduced the term into musical language. In this case, it was the secretary of France's Academy of Fine Arts doing the scolding, and it was the young composer Claude Debussy who was getting his knuckles rapped. Debussy had won the academy's prestigious Prix de Rome scholarship in 1884, and in 1887 he was applying for it again, this time with an orchestral piece called ''Printemps.'' The piece made for a nice try, the secretary commented in his formal letter of rejection. But it was hardly a work for which the prize could be awarded. ''Monsieur Debussy does not lapse into banality, nor is he platitudinous,'' the secretary wrote. ''On the contrary, he has a pronounced tendency - too pronounced - toward an exploration of the strange. One has the feeling of musical color exaggerated to the point where it causes the composer to forget the importance of precise construction and form. It is to be strongly hoped that he will guard against this vague impressionism, which is one of the most dangerous enemies of truth in works of art.''

Even then, of course, this ''dangerous enemy of truth in art'' was getting the upper hand.

Prodded by Leroy's seminal condemnation of the ''Sunrise'' canvas, Monet and his colleagues soon enough accepted the challenge and proudly - defiantly - billed themselves as ''impressionist'' painters, and it was not long before their dealers began using the tag as a marketing device. As noted, the word was slower to catch on in musical circles, but as early as the mid-1890s it was fairly peppering concert reviews in France and Belgium. Once breathed with an air of derision, ''impressionism'' by the end of the century had evolved into a kind of puffery, a chichi slogan as fashionable as the art to which it referred.

Debussy hated it. He called it ''a useful term of abuse,'' a slippery bit of jargon whose only justification was that it made it easier for workaday newspaper critics to do their jobs. Since Debussy is the composer whose name remains most solidly linked with the concept of impressionism in music, his complaint is worth considering. When applied to music, what, if anything, does the term mean? And if it is meaningful, does it have any real connection - aside from the obvious link of time and place - with the paintings on display at the Art Museum?

It may well be that, today as in Debussy's day, ''impressionistic'' is an adjective over-used by reviewers too lazy to describe a piece of music in detail. Still, the term does have meaning, albeit more in regard to the composer's intention - realized or not - than to the work's specific melodic or harmonic content.

Persons who recently scored high marks in a music appreciation class might argue with me about this. They are likely to have been taught that impressionistic music has a certain sound to it, a sound - rhythmically fluid, richly colored, only vaguely tonal - remarkably like what one hears in such works of Debussy as ''La Mer,'' the ''Nocturnes'' and ''Images.'' But that is only because Debussy is so strongly identified with the more or less official Impressionistic movement. The works just named are indeed impressionistic, but the same sound can be heard in his ''String Quartet'' or his ''Rhapsody for Clarinet and Orchestra,'' works that are not at all Impressionistic. By the same token, there are works from early in the 19th century as well as works from our own time that, even though they have practically no stylistic bond with Debussy's music, are as impressionistic as any ever composed.

It is a matter of attitude on the part of the composer, and sometimes a matter of attitude on the part of critics and audiences who have second-guessed the composer and transformed his music into an artistic ''object'' quite different from what it was originally.

Music dictionaries proved to be of not much help to me as I wrestled with the impressionism questions. Instead of a definition, they tend to give only examples; invariably they point to Debussy, and they conclude that impressionistic music is music of the sort produced by Debussy and his like-minded contemporaries. More useful were the English language dictionaries. Most of them, not surprisingly, define '''impressionism'' as music or art along the lines of Debussy's or Monet's. But all of them deal adequately enough with ''impression.''

According to Webster, the word means, among other things, ''an indistinct notion or recollection.'' And therein lies the key.

It's easy to tell the difference between paintings that are distinct and indistinct. In the one case the imagery is clear, and in the other it is not. The paintings of which Louis Leroy approved were of the former variety, and that's why Monet's ''Sunrise'' - so soft-edged as to be barely recognizable as a picture of a dawn - was so bothersome to him.

But how, just from listening, can one tell a ''distinct'' piece of music from its opposite? Is it not the nature of music to be indistinct? A composer can, of course, make an onomatopoeic gesture that clearly imitates the sound of birds twittering, trains crashing or some other everyday sonic phenomenon. This, however, is rare. Even the most blatant examples of so-called program music ''refer'' to the real world only by analogy. We don't really ''hear'' a battle taking place when we listen to, say, Tchaikovsky's ''1812 Overture'' or Beethoven's ''Wellington's Victory''; we just hear one martial air and then another, with a lot of noise in between; our imaginations take care of the rest, but only because the composer has prompted us - through verbal cues - to do so.

Impressionism is a form of program music, distinguished from that broader category only by the relative vagueness of whatever it is that is allegedly being depicted. And it is definable, in my opinion, only by its extra-musical embellishments.

If I wrote a five-minute piece and titled it ''Sonata No. 472 1/2,'' it would be an example of abstract music. But if I merely said that it represented - block by block - the full range of urban drama I encountered on my daily drive to the office, then it would be an example of program music. If I said that the very same piece expressed the idea of moonlight as filtered through a St. Louis sky thick with humidity, then it would be an example - for better or worse - of musical impressionism. If   I meant for the piece to be heard as an abstract composition, and if some critic 100 years from now decided to call it the ''Moonlight Sonata,'' then the piece might well become - over my dead body - part of the impressionistic literature.

In deciding whether a piece is impressionistic or not, what the composer has to say about it obviously counts for a great deal. But composers should not always be taken at their word.