isr.co.ro : glossary arj A compression program for MS-DOS (an operating system) and Windows computers developed by Robert Jung of ARJ Software. It's often used for posting multi-disk files to Usenet newsgroups. As with most such commercial compression systems, you have to license the compression program, but you can download and use for free any of the matching decompression programs for various platforms. The file format produced by this program. Files of this type are normally marked with a suffix of ".arj" on the first part, and then with an ".a01," ".a02," and so on for up to 99 other parts.