What is a wine bar? Having spent far too much time, and money, in the wine pubs of London, I feel well qualified to proffer an opinion. In the simplest definition, a wine bar, or wine pub, is a public drinking place which serves wines, by the glass and bottle, to the exclusion of almost any other type of alcohol. From that base, variations can be added. Much depends on the location and the desired customers: wine selection, pricing policy, opening and closing hours, decor and atmosphere, the expertise of the staff and type of food, if any.

In London, the most successful wine bar city, the phenomenon spread in the early '70s. There had always been a handful of wine pubs, but most were in the City or near Fleet Street. They were masculine in atmosphere and patronage. You may recall the long-standing feud between the women journalists and El Vino's, the Fleet Street pub that does not permit women to stand at the bar or to buy their own wine and sandwiches. So much for emancipation. Today, you can find a wine pub in London to suit your mood, pocket and wine and food preferences. It's my favorite way of lunching in London: a glass or two of wine and a light, inexpensive meal.

The wine bar concept is no longer unique to London. I have been into wine bars in Toronto, San Francisco, New York and Cape Town. Like London, there's no rigid model in these cities. San Francisco's London Wine Bar has a retail section on the same premises, permissible in California but not here.

In Washington, there is a welcome trend towards serving better wines by the glass, but, by my definition, this doesn't transform a restaurant or a regular bar into a wine bar.

Of those that qualify, the Carlton Wine Bar, one of the first, is consistent in the high quality of its wines and service. In a quiet, club-like room, in the Sheraton-Carlton Hotel, it is open from 4 p.m., weekdays only. Cheese and pate are available. Prices can be higher than other bars because one is paying for the sophistication and the overhead of a luxury class hotel.

Chez Maria is conversely elementary in design, in the sparsely decorated basement of the Georgetown Vietnamese restaurant of the same name. It is open from 5 p.m. and serves cheeses to accompany the weekly wine specials.

Suzanne's, near Dupont Circle, is a comparative newcomer. Despite excellent wine prices and the flexibility of chalkboard listings of the wines by the glass, it is still in a formative stage. Open for lunch and evening meals, it veers towards being a cafe rather than a pub.

The Wine Cellar, in the Maryland Inn, Annapolis, is a real, subterranean cellar. It lists all the wines available elsewhere in the Inn, at the same refreshingly low prices, plus weekly specials. It opens at 4 p.m. and serves light meals.

Since most people go to a wine bar to enjoy themselves, I don't want to belabor what should be a pleasant activity, but there are a few principles with which my basic definition could be embroidered. Principle One: The wine is the key factor. The staff of a bar should have a good knowledge of wine, or, at least, of the wines on their own list, without necessarily demanding an equal reverance for wine from their patrons. Principle Two: For the more serious, a bar offers a chance to expand interest and knowledge by experimenting with wines by the glass. Principle Three: One should not pay more for the same wine in a bar than in a full-scale restaurant. However, when comparing prices, keep in mind that glass sizes may vary.