Americans long wedded to pizza have begun to wonder how to distinguish between their favorite pie and the focaccia that now turns up everywhere. Pizza, after all, has been the beloved, second only to hamburger on the nation's menu. Focaccia, the darling of health-minded, status-conscious epicures, is the interloper.

Or is it?

"Technically, focaccia is pre-pizza," said Larry Goldberg, who operated pizzerias in New York and Kansas City for more than two decades. He is now impresario of Goldberg's Pizzeria in Prairie Village, Kan., a suburb of Kansas City, and is a scholar of his medium.

"Centuries ago, they put flat bread under the fire," he said. "Technically, what 'focaccia' means is 'under the fire.' Then, they threw on some olive oil and some herbs, and there you had it: focaccia, or to my way of thinking, pre-pizza."

By any name, the flat bread, embossed with oil and sprinkled with rosemary, rubbed with black olives, tomato sauce or pesto, appears by the tower at farmers' markets in cities like San Francisco, Minneapolis and New York. In specialty shops, focaccia (pronounced foh-KAHT-cha) has practically eclipsed baguettes. And in groceries around the country, the fresh rounds -- sometimes called focaccia and sometimes "bread shells" -- are beginning to challenge frozen pizzas. At home, the cook with a yen for a simple, soul-satisfying meal for company doesn't know whether to look under "P" or "F" in the cookbook index.

Most experts concur with the Goldberg theory of pizza evolution, but few agree about what distinguishes focaccia from pizza today. Carol Fields, author of "Italy in Small Bites" (Morrow, 1993), says that in general, dough topped with tomato sauce or cheese is pizza, while dough with the toppings kneaded into it is focaccia.

This distinction is in accord with the writer Calvin Trillin's experience. "I've rarely seen focaccia with anything on it, unless it's at the bottom of the bag," he said. But Mrs. Fields fears that this single distinction is simplistic. In addition to toppings, she said, one can use thickness and temperature to distinguish between the two forms of bread. In Italy, she said, pizza crust is generally less than a half-inch thick, and fo caccia is about three-quarters of an inch. There, pizza is never served cold, at least not in polite circles, while focaccia is intended to be served at room temperature.

Nevertheless, cold pizza is not focaccia, Mrs. Fields said. A pizza pie is an immutable entity, she intimated, while focaccia can be turned into many things -- warm appetizers, stuffed sandwiches or the base for a one-dish dinner -- and there begin the semantic nightmares.

"You have to thread your way through a myriad of turfs," Mrs. Fields said.

Despairing, perhaps, of distinguishing between the two bread forms, General Foods USA made up a new word for its bread shell.

"Rather than confuse the public, somebody thought of calling it 'Boboli,' " said Nan Redman, the company's spokeswoman, in White Plains. "It's one of those made-up words. I don't believe there is an Italian translation for Boboli." Similar products are called "pizza bread," "pizza shell," "crustino" and, in an all-inclusive capitulation to consumer confusion, "focaccia pizza bread."

These labels are of little help to the home cook seeking recipes and guidance through the thicket of Italian breads that can make a meal. Eileen Weinberg, owner of Good and Plenty to Go, a food shop in Manhattan, says the marketing labels have increased confusion about the two breads. "People are selling pizza as focaccia and focaccia as pizza," she said before offering her own distinction.

"Basically, focaccia connotes bread with a little topping and pizza connotes topping with a little bread," Ms. Weinberg said. "People interested in eating light want focaccia, and it can be topped with vegetables, olive oil and herbs. Meat and cheese toppings turn the same thing into pizza."

Pizza authorities also cite several differences between pizza dough and focaccia dough. Traditionally, pizza is a concoction of yeast, flour and water, while focaccia also contains olive oil and, often, rosemary or other savory herbs. Pizza dough, Mrs. Fields said, "is a little more difficult to make, as it must be stretched or rolled thinner than focaccia, which is pressed into shape with the fingertips."

Even if it is a less highly evolved member of the pizza family, focaccia has more snob appeal than pizza.

"The difference between the two is about a buck," said Gregory Heinz, a customer at Ray's Pizza in Greenwich Village. "The focaccia cost more than the pizza, and you can't buy the focaccia by the slice."

Exactly, said Michael Stern, co-author with Jane Stern of "Way Out West," a celebration of cowboy culture and cooking to be released this month by HarperCollins.

"Pizza is too popular to have snob appeal," he said. "You cannot invite fancy people to your home and serve pizza. It would be like serving pigs-in-a-blanket. Focaccia to the rescue! Even pepperoni focaccia has more class than pizza."