Unix program whose primary purpose is the low-level copying and conversion of files. Some have suggested that it stands for Copy and Convert, but since "cc" was already in use as the code for the C Compiler, "dd" was used instead; in any case, DD is a shorthand for "data definition" in IBM JCL (to which the command's syntax bears a strong resemblance) and the name and syntax of the command is generally presumed to be a bit of gallows humor.