A file in a compressed format. Many operating systems and applications contain commands that enable you to pack a file so that it takes up less memory. For example, suppose you have a text file containing ten consecutive space characters. Normally, this would require ten bytes of storage. However, a program that packs files would replace the space characters by a special space-series character followed by the number of spaces being replaced. In this case, the ten spaces would require only two bytes. This is just one packing technique -- there are many others. One disadvantage of packed files, however, is that they can be read only by the program that packed them because they contain special codes.
Some modems automatically pack data before transmitting it across communications lines. This can produce faster communication because fewer bytes need to be sent. However, the modem on the receiving side must be capable of unpacking the data.
Packing is often referred to as data compression, particularly when it involves data communications.