1. <storage> double density.
2. <database> data dictionary.
A Unix copy command with special options suitable for block-oriented devices; it was often used in heavy-handed system maintenance, as in "Let's "dd" the root partition onto a tape, then use the boot PROM to load it back on to a new disk".
dd had a distinctly non-Unixy keyword option syntax reminiscent of IBM System/360 JCL (which had an elaborate DD "Dataset Definition" specification for I/O devices). Though the command filled a need, the interface design was clearly a prank.