When you use the word "phenomenon" in the auto business, it's generally understood you're talking about the Oldsmobile Cutlass.

Barring something extraordinary, the General Motors Corp. intermediate later this month will be proclaimed the best selling model during calendar 1976, handily topping its closest competitor, the full-sized Chevrolet.

The basic reason the Cutlass is viewed as a phenomenon is a simple one: It differs only minimally from the Chevrolet Chevelle, the Buick Century and the Pontiac LeMans, yet has outsold them consistently and dramatically.

Late last month, John M. Fleming, Oldsmobile assistant general sales manager, said, "It's going to end up the year as the number one nameplate in the industry, for one sure." That will be a first for any intermediate car.

Through the first ten days of December, Cutlass sales were 488,500, roughly 54,000 ahead of the big Chevrolet, Fleming said. "I don't think they can perform a miracle in 20 days," he added.

Except for a blip in the desperate year of 1974, Cutlass sales have risen steadily for more than five years. At the same time, the car's share of total intermediate new-car sales in the U.S. rose from 13.9 per cent in 1971 to almost 18 per cent of a much bigger number of intermediates sold now.

Starting with the September introduction of the "downsized" Pontiac big cars, sales of the Cutlass and other GM intermediate cars have been watched very closely here.

GM's new 1977 big cars such as the Chevrolet Impala and Oldsmobile Delta 88 are roughly the same size overall as its intermediates such as the Chevelle and Cutlass. They have roughly the same curb weight overall length and wheel base. They even get the same miles per gallon.

But the big cars are substantially roomier inside, and are restyled as well.

Further, the price differential between GM's intermediates and fullsized cars wasn't altered, a move that caused some dismay GM's competitors.

Fleming acknowledge some softening of Cutlass sales was anticipated with the advent of the smaller fullsized Delta 88.

But "it sure doesn't look like that now," he said.

Given the minimal differences between the Cutlass and GM's other intermediates - the Chevelle, LeMans and Century - the car's continuing success has a touch of mystery to it. The Chevelle is sold by many more dealers, for instance.

Asked what is behind the car's success, Fleming offers a variety of reasons.

"A lot of the explanation is that over a period of time, an awful lot of people have moved down in size, from the traditional family-sized big Ford and Chevrolet to intermediates.

"They found they could buy an Oldsmobile for not much more than the Ford or Chevrolet," he said. The Oldsmobile name is a step up, and they change feel pretty good about the change, he said.

Also, Oldsmobile "pays real good attention to quality," and the division hasn't change the names of its intermediate like some of others have. "Every car we have in that class is called a Cutlass, from the Cutlass Supreme to Cutlass Broughams," he said.

The car has a superior resale value, and that has attracted fleet buyers, Fleming added.