IF THIS WERE a history of decadence in all its gaudy manifestations throughout the ages it would both fill an oddly yawning gap in cultural history and provide the source for many a Fellini movie to come.

Alas, as its subtitle warns us, it is merely "the strange life of an epithet" - at best a history of the concept of decadence; at worst a matter of word-juggling better suited to a brief essay than a book of even this modest length.

Decadence to Richard Gilman, distinguished critic and author of The Confusion of Realms and The Making of Modern Drama, "is not a fact but a value judgment, a category of belief or opinion." It is, further, "a scarecrow, a bogy, a red herring."

What has Gilman especially exercised is that today it has become a "perverse honorific." In our schizophrenic culture, true trendiness lies either in blatant health seeking (jogging, swilling Perrier) or in reveling in such "decadent" pastimes as self-exposure at Studio 54 or painting one's rumpus room puce.

Even our mythic sense of decadent places is merely an attribute of our "vulgar" minds. Gilman lists them: "ancient Rome, Alexandria, and Byzantium . . .Paris of La Belle Epoque, London of the Yellow Nineties, Istanbul, Berlin in the twenties, Hollywood, Fire Island."

Without going much into the Fire Island, Gilman questions the decadence - in our sense of the word - of some of these cultures on the basis of their productivity. Like a Victorian travel guide averting our attention from the naughtier frescoes at Pompeii, Filman neglects the risque Weimar Republic of Christopher Isherwood for such substantial achievements of that time and place as the Bauhaus.

It was the Nazis, of course, who branded every significant work of the period as "decadent art" and then went on to demonstrate their own psychic and moral health at Dachau and Auschwitz.

None of these loose applications of "decadence" has anything to do with the real article, which according to Gilman involves "neurasthenia and sexual perversity; boredom combined with exquisite refinement; nostalgia for the corrupt; fascination with the splendors and despairs of ancient cultures," etc. What does the habitue of Studio 54 know or care about ancient cultures?

The closest we come to an understanding of decadence today is Mick Jagger proclaiming that Paris is "conducive to having a good time. It's like Rome in that sense - decadent - which is nice, but . . . ."

But if Rome really was decadent, Gilman asserts, its fall was largely caused by outside forces. Like all civilizations, Rome existed in a continuum of history, the dynamics of which are too complex to allow of such glib concepts as "decadence."

Yet Gilman ignores the acute Oscar Wilde pronouncement on the U.S. as the only country in history to have passed from a state of barbarism to a state of decadence without ever having gone through a state of civilization.

Wilde otherwise figures large in this study, as well he should, along with the French and British decadents of the second half of the 19th century.

What really arouses Gilman's ire, though, is the word's promiscuous use in our time to describe everything from a "decadent" stage performance (Variety ) to the "moral and systemic decadence" that led to Watergate (Sen. Jacob Javits).

The New Yorker, Women's Wear Daily and The Village Voice are similarly taken to task for misusing the word every time they want to hype up an otherwise limp perception, and it's good to see such high-minded bores as Oswald Spengler and John Gardner get their lumps as well.

Media-hype is nothing new, though, in the history of decadence, with Gautier beating the drums for Baudelaire and J. K. Huysmans doing a similar public-relations number on the painter Gustave Moreau, way back in the 19th century. When Wilde, perhaps the most skilled publicist of decadence, undertook the same task for his mentor Walter Pater, he sorely embarrassed that reclusive Oxford don.

Yet with all these insights, the sum effect of Gilman's well-reasoned book is one of petulance rather than profundity.

Words have always been misused - never more, to be sure, than in these anti-literate times - so to single out the decadence of "decadence" for such heavy treatment itself seems perverse. Just consider poor "gay." And weren't the Victorians abusing the word "abuse," for instance, in their morally pejorative term "self-abuse" for what Woody Allen much more accurately defined as having sex with someone you really love?

And speaking of decadence, the book itself represents a sad falling away from publishing standards of the past.

The names of Hugo von Hofmannsthal, Jean Lorrain and Osbert Burdett are consistently misspelled; Voltaire's home in Ferney is converted to the Italian physicist Fermi, and, so obsessed is the author with his pet word that Arthur Symon's famous work The Symbolist Movement in Literature becomes The Decadent Movement in Literature.

Professors out to rap our knuckles for slovenliness, even as engagingly as Gilman does, should do their own homework first. CAPTION: Illustration, Aubrey Beardsley's drawing of Oscar Wilde