It is the logical name for a computer smaller than the microcomputer, which is smaller than the minicomputer.
(The minicomputer is called "mini" because it was a lot smaller than the original (mainframe) computers.) More technically, it is a computer whose fundamental parts are no bigger than a few nanometers.
For comparison, the smallest part of current state-of-the-art microprocessors measures 45 nm as of February 21, 2007. No commercially available computers that are named nanocomputers exist at this date, but the term is used in science and science fiction.
There are several ways nanocomputers might be built, using mechanical, electronic, biochemical, or quantum technology.
It is unlikely that nanocomputers will be made out of semiconductor transistors (Microelectronic components that are at the core of all modern electronic devices), as they seem to perform significantly less well when shrunk to sizes under 100 nanometers.
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