vt. 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.
However, note that ground-to-space communications has its own usage rule for this term. Space-to-earth transmission is always download and the reverse upload 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.