The definition of PDA

A personal digital assistant is a small battery-powered computer intended to be carried around by the user rather than left on a desk. This means that the processor used ought to be power-efficient as well as fast, and the OS ought to be optimized for hand-held use. PDAs typically have an instant-on feature (they would be useless without it) and most are grayscale rather than color because of battery life issues. Most have a pen interface and come with a detachable stylus. None use mouses. All have some ability to exchange data with desktop systems. In terms of raw capabilities, a PDA is more capable than an organizer and less capable than a laptop (although some high-end PDAs beat out some low-end laptops). By far the most popular PDA is the Pilot, but other common types include Newtons, Psions, Zauri, Zoomers, and Windows CE hand-helds. By far the fastest current PDA is the Newton (based around a StrongARM RISC processor). Other PDAs are optimized for other tasks; few computers are as personal as PDAs and care must be taken in their purchase. Feneric's PDA / Handheld Comparison Page is perhaps the most detailed comparison of PDAs and handheld computers to be found anywhere on the web.