A fad, by classical definition, is a short-time thing, a craze that may or may not be a trend.

Since 1988 is the 30th anniversary of the most widespread fad in American history, the Hula Hoop, it seems a fitting time to look at faddism and what makes it tick, or in the case of the "new" Hula Hoop, shoop-shoop -that's the sound caused by a steel ball bearing inserted in the hollow tube.Author Richard A. Johnson in "Fads," the basic book on the genre, has dubbed the Hula Hoop "the un-disputed granddaddy of American fads."

That pretty well eliminates fads that swept America during the first 250 years of its history, including such things as stitchwork samplers, hair wreaths, and silhouette portraits.

Nevertheless the trend to increased faddism really began with the birth of the Hula Hoop in 1958, a sort of West Coast version of a bamboo hoop used in Australian gym classes. At the height of the hoop craze in 1958, its manufacturer, Wham-O of San Gabriel, Calif., was producing 20,000 Hula Hoops a

day. Since 1958, there has been the pet rock, frisbee, CB radio, Wacky Wall-walkers, mood ring, Rubik's Cube, Trivial Pursuit, and Silly Putty, not to mention Reebok walking shoes, white wines instead of liquor cocktails, miniature Japanese trees, and kiwi fruit.

Some, but not all of these have been short-time items and will stay dead, but there is the possibility that some will have a revival, as the Hula Hoop is having this very instant, just as Teddy Bears and argyll design socks have made significant comebacks in the past few years.

Wham-O is planning the first national television promotion of the Hula Hoop, a $5 item, in more than a decade and a national video talent search for the best hula hooper. Sales of the hoop, which hit 100 million by 1960, had dropped to one million in 1986 but moved up to 2 million last year.

"There is every indication that sales will be 4 million this year," said Don Roddick, sports promotion director of Wham-O. "It's very much a spontaneous buy, often by people who enjoyed hooping when they were 10 years old and now have 10-year-old children of their own to buy for. It's a phenomenon, surely- a fad that hasn't died.

"The word fad has the implication of one time only, and the pet rock was a real fad in this sense, Roddick said. "I don't think we'll ever see a renewal of interest in pet rocks. I hope they've found their way into the garden. But then we have the Frisbee, a fad that a whole sport has grown up around with an international federation of frisbee clubs and hundreds of tournaments." The Futures Group, of Glastonbury, Conn., a consulting firm that forecasts trends for both public and private clients, said fads spread because the public decides they are "in," which means popular with trendy people.

Fads fill the need to belong and could even fill some newly perceived need, as the CB radio did. Fads spread faster if they get media attention.

"Timing is everything," said Ted Gordon, a Futures Group spokesman.

"The innovator or original manufacturer has to decide when it is going to take off in order to plan greater production capacity," Gordon said. "The product must be given a perceived value to the purchaser. Once that is done, the big build-up may be done by someone other than the innovators.

Gordon said fads fall into two categories- "pure fun and functional."

Ken Hakuta became a multi-millionaire in the past six years as the developer of the Wacky Wallwalker, a gummy "bug" that walks down walls. He is the nation's major promoter of fads, known in the trade as "Dr. Fad."

"Useful ideas have their place in the world, but these are more innovations of existing products than new products. In general they have big companies and substantial advertising behind them," he said.

"It's useless items- the gizmos- that sweep the country all on their own, the real fads. They have no advertising, no marketing plan, no big corporation, no technological innovation. They are original products and they don't change throughout their short life. The best fads are like jokes. You laugh when you find out about them."

Hakuta must be right.

It's hard not to laugh when you try hula hooping the first time. Even the umpteenth time, you've still got a smile on your face.