A Navi is a handheld computing device concept from the anime series Serial Experiments Lain. It looks, and works almost identical to the Palm Pilot, only with 5+ years advanced technology.
It is used by Lain, and her friends mostly for communication through email, and instant messages. Lain also uses it as a development platform for her massive computer at home.
The original idea for a similar "Navi" was from Apple computers. Since Serial Experiments Lain has so many apple references, why not include another.
"Navi" is most likely a contracted form of "Knowledge Navigator." John Sculley, the CEO of Apple from 1983 to 1993, described the Knowledge Navigator in his book titled Odyssey (1987). The Knowledge Navigator was his vision of Apple computers in the 21st Century. Accurately predicting many of the changes eventually brought about by the World Wide Web, the hypothetical Knowledge Navigator would allow people to communicate with each other from anywhere in the world, would be connected to a vast shared hypertextual (Sculley cites Ted Nelson in reference to hypertext) database of information (like the Web or the Wired), and would also utilize intelligent agents or artificial intelligences to actively search out information of interest.