Superstores is what people in the food business call their newest invention, and there aren't any of them in Washington. Yet.

Superstores are coming, soon, however, as part of Safeway's $22 million expansion plan in the metropolitan area. If Giant Food doesn't get some, too, it will be only because of definitions, because Giant's biggest stores are almost "super" by the industry's standards.

So how is a superstore different from a supermarket?

It is bigger. And it sellsss more things, a lot more than just food.

Like television sets. You already can pick up a little Sony at the Safeway store in Olney, where the chain has its biggest nonfood department in this area, the prototype for superstore expansion here.

The Olney store is also the only store Safeway has on the East Coast that accepts bank credit cards. Mastercharge and Visa can't be used for groceries, only nonfood items, and purchase minimum precludes small transactions. Food and charge cards don't mix because the service charge to merchants on bank cards is bigger than typical supermarket profits.

As the biggest food chain in the country, Safeway last year earned 91 cents on every $100 of business. On $11 billion of sales, it earned $102 million in 1977.

These slim food profit margins are the reason companies such as Safeway are trying to increase the portion of nonfood items that go through their checkouts.

Television sets and microwave ovens carry the biggest tickets in superstores, but there are lots of other smalll electric items, cameras, watches and gifts avaiflable from a serviced counter.

Housewares, a greatly expanded health and beauty aids department, auto supplies, small hardware items, greeting cards and selected softgoods - such as inexpensive canvas shoes and towels - are in the mix.

Safeway's superstore formula calls for 40,000 square feet of selling space - roughly one-third to one-half more than conventional stores - with the usual meat, grocery and produce departments, the expanded nonfoods section and one or more of what food marketing people call "boutiques."

In some areas, the "boutiques" may be a wine and cheese shop, a fast food take-out or branch bank, but locally they most often will be delicatessens, bakeries or pharmacies.

Giant Food has been agressively turning its health and beauty aids departments into full-fledged pharmacies for several years, and now has 41 of them. (Market estimates place Safeway first in the Washington market, with 33 percent of the food business to Giant's 30. In the Baltimore-Washington market, Giant is first with a 24 percent share to Safeway's 22.)

The name of the game, of course, is "one stop shoppint" - buying everything at the food store, a concept that has not been notably successful in this country.

Superstores, in fact, are second or third-generation mutations of giant stores called "hypermarches" that were born in France, emigrated to Canada, and died in Chicago as a noble experiment of the Jewel Companies.

The first successful "super-stores" were opened by South-western independent supermarketers as a way to one-up chains such as Safeway. Safeway is one of those companies that has a reputation for never doing anything first, but nearly always doing it better.

Superstores cater to super-shoppers, the people who spend $70 a week or more in grocery stores. That is about 42 percent of the 2.1 million people in the greater Washington area, according to one study of local consumers. One-stop shopping also is aimed at working women and in Washington that is a majority of adult females.

Long-time followers of food marketing such as Robert Rossner, editor of Convenience Stores magazine, see superstores as the evolutionary product of the breeding of retail giants. It is more efficient for a big chain to run 1,000 super stores than 2,000 conventional markets because overhead is lower.

But as superstores proliferate, says Rossner, so do neighborhood convenience stores running between the legs of the giants, serving the customers who don't go to their grocery store to buy a television set.