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.