1. [techspeak] To transfer data or (esp.) code from a larger `host' system (esp. a mainframe
) over a digital comm link to a smaller `client' system, esp. a microcomputer or specialized peripheral. Oppose upload
2. [jargon] To fetch data (especially large relatively standalone pieces of data like files and images) over the wire from a remote location.
However, note that ground-to-space communications has its own usage rule for this term. Space-to-earth transmission is always `down' and the reverse `up' regardless of the relative size of the computers involved. So far the in-space machines have invariably been smaller; thus the upload/download distinction has been reversed from its usual sense.