Maybe nobody was looking, but the word spinster seems to have disappeared from the vocabulary at about the time we got such new expressions as women's lib, feminism and (much beloved by elderly male politicians) bra-burners.

The disappearance was sort of a shame, really. "Spinsters" was a helpful expression, colorful, evocative, useful in describing someone in one word. How, for example, are we now to describe Emily Dickinson? Louisa May Alcott? The female subjects of all those Norman Rockwell covers for the Saturday Evening Post?

Spinsters used to be as American as apple pie; now both have been replaced by Pop Tarts. But what was a spinster, anyway? An office colleague, female, has just supplied a definition: a woman of a certain age, unmarried, with her hair in a bun. Katherine Hepburn in "Summertime," for example, played a spinster.

Spinsters passed their time in traditional fields. They were librarians, high school geometry teachers, sold greeting cards from their living rooms, entered decorative gourds in the country fair, wore flower arrangements on their hats, drank tea, thought every little boy should be overjoyed to mow their lawns or shovel their snow for 10 cents . . . and, if they lived to be old enough, were generally referred to by others in the neighborhood as the Cat Lady

Spinsters were a part of American folklore as recently as 1964, when they were often described as little old ladies in tennis shoes who, according to your political persuasion, (A) thought Barry Goldwater was a left-leaning liberal or (B) thought we should unilaterally ban the bomb.

When they were not engaged in extremist political activities, however, they were okay people. They went out of their way to help others. Many, for example, crossed at busy intersections several times a day as a favor to the Boy Scouts. Others mounted expeditions to Venice, Italy, to demonstrate in favor of the cat population, joined or resigned from the Daughters of the American Revolution (in either case for impeccable reasons), collected recipes for Lime-Carrot Holiday Mold Cake and knew how to keep geraniums alive down in the basement all through the winter.

And now there no longer are any spinsters. Well, when was the last time you used the word, or thought of it or thought of anyone in connection with it?

The former spinsters can't all be gone. Women are supposed to outlive men by 5 to 15 years on the average these days, and God knows there are still enough bachelors around (.92 or thereabouts for every single woman). But the word itself has sort of evaporated.

Maybe it's because we don't need it any longer. Spinster was a word that said all sorts of things in addition to what it apparently said. It said (A) this woman hasn't reproduced herself; (B) she is repressed; (C) she's odd and eccentric; (D) she's lonely by choice, and that's an affront to the loneliness approaching for all of us; () she-gasp!- doesn't need a man and (F) she could never afford to live in that house if she hadn't inherited if from her father, who was the meanest man in town and foreclosed everybody's mortgages, so her situation serves her right.

We don't seem to feel or think those kinds of things about single women any longer. The papers and TV programs are full of single women who are running health clinics, acting as agricultural advisers in India, designing office buildings, running for public office or even living in open marriages. It was hardly a surprise to learn recently that only 7 percent of American families reflect the Norman Rockwell traditional-model home unit (father the breadwinner, mother the homemaker, Dick and Jane out in the backyard with Spot).

Spinsters used to be . . . different. Now there's a premium on difference. Spinsters used to be thought a little selfish because they did for themselves. Now narcissism is the subject of half the books on the best-seller list. Spinsters used to be thought of as older than other people, but now the World War II baby boom population bulge is aging toward what used to be called a "certain age".

Most of all, spinsters used to be thought odd because they didn't define themselves in terms of their relationship with a man. These days, more and more Americans define themselves in any way they damm well please. The day is coming when those collections of classic Saturday Evening Post covers are going to come with footnotes, explaining the odd, strange yes, even bizarre-ancient American folkaways that dear old Mr. Rockwell was illustrating.