“I think there may be just three left on Fifth Avenue still operating as single-family residences,” said Sharon Baum, a senior vice president of the Corcoran Group, who sold the most flamboyant of the bunch, the Duke-Semans, at 1009 Fifth Avenue opposite the Metropolitan Museum of Art, for the Duke family matriarch in 2006. “It had been in the family for more than a hundred years; the only reason she sold it was, no one in the family wanted to move to New York City to live in it.”

The price was $40 million, and five years later Carlos Slim, one of the wealthiest men in the world, bought it for $44 million. With 19,000 square feet and eight levels, the house is generally acknowledged to be “the most important Beaux-Arts residence in New York,” Ms. Baum said, adding, “But it’s all in the eyes of the beholder.”

When it was on the market, Ms. Baum fielded inquiries from potential buyers wondering “how many acres there were” and whether “it had bulletproof glass in the windows.”

Wendy Maitland, a senior managing director of Town Real Estate, disagreed with the rule of thumb that width and frontage are default determinants of mansion status. “There can be a mansion that is 20 feet wide, if it is done in a specific way and possesses gravitas,” she said. “You can’t just define a mansion by width or square footage: it has a private street-level entrance and is a stand-alone building, usually built in the late 1800s, typically limestone with five levels and multiple fireplaces and outdoor spaces, and the rest are, in my humble opinion, not actual mansions.”

Not all developers of luxe condominiums have jumped on the mansion bandwagon. At 18 Gramercy Park South, a genteel makeover of a Salvation Army boardinghouse by the team that brought 15 Central Park West (and Fifth Avenue pricing) to the West Side, there was no hesitation about what to christen the 3,746-square-foot, $9.53 million, two-bedroom three-and-a-half-bath residence with its own Irving Place entrance. Despite 17-foot ceilings and park frontage, it is a maisonette, period, and the least expensive unit.

“A mansion is a stand-alone home or very important town house,” said one of the condo’s developers, William Lie Zeckendorf. “A maisonette is a town house contained within an existing building. A maisonette benefits from the building’s services and staff, whereas a mansion needs to have its own services and staff.”

Having sold many of Manhattan’s most opulent establishments, Ms. Baum finds humor in the sudden popularity of mansions, in whatever form. “Don’t forget about the mansion tax that has to be paid on any home that costs more than $1 million,” she said. “If you use the mansion tax as your definition, it means practically anybody who buys a home in Manhattan in this market is buying a mansion.”