“It’s unclear what that gets you,” he said. “A couple of less pounds?”

Mr. Baker added that he generally considers computers with screens as large as 12 inches not in the ultraportable category. He draws the line at those under 10 inches.

While a precise definition of ultraportable varies, the broader attributes of the category are as easy to discern as telling a sports car from a sport utility vehicle. Standard notebook computers generally have screens that measure 15 inches or so when measured diagonally (17-inch screens are also popular, many retailers report). Many computer makers, including Toshiba, Sony and Hewlett-Packard, classify ultraportable notebooks as those with screens measuring 12 inches or less.

Computer makers note that when laptops have smaller screens, the rest of the computer must adhere to a proportionality that usually forces the keyboard  as well as the overall size of the computer  to be smaller than standard. In turn, the body thins and the computer gets lighter. Many ultraportable notebooks weigh around four pounds or less, while an entry-level Dell Inspiron 1501 notebook with a 15.4-inch screen, in contrast, weighs 6.19 pounds.

In past years, packing components so closely together to achieve what computer makers call a smaller “footprint” encouraged many laptop manufacturers to use less powerful microprocessors than those in larger computers to reduce heat and not overtax smaller batteries. But advances in microprocessor design and cooling systems have significantly reduced the need for such striking compromises, especially in the higher-priced ultraportable notebooks, experts say.

“We are excited about the fact that laptops can now pack more power into a smaller, more lightweight and energy-efficient package,” said Karen Regis, manager of the mobile platforms group at Intel.

Or as Mr. Abary of Sony put it, “Miniaturization is an art of engineering.”

Showcasing such engineering, he said, is the Vaio VGN-TXN15 by Sony, released this spring. It is part of the company’s TX series of full-featured, ultraportable notebooks. The TXN15 has an 11.1-inch screen in a wide (16:9) aspect ratio and weighs 2.8 pounds. Mr. Abary said much of the notebook’s economy of size, weight and strength was managed with innovative engineering as well as the use of carbon fiber for its chassis.

The Vaio TXN15, which costs $2,300, is powered by a 1.2-gigahertz Intel Core Solo Ultra Low Voltage microprocessor and, depending on use, offers a standard battery life of 5 to 11 hours, Sony engineers said. The TXN15 also features a fingerprint security sensor and is equipped with sensors to protect the hard drive if the notebook is dropped or severely jostled.