(1) (Open System Interconnection) An ISO standard for worldwide communications that defines a protocol framework in seven layers. OSI functionality exists in all communications systems, although two or three layers may be incorporated into one (see OSI model). Control is passed from the top application layer to the bottom layer, out over the network to the bottom layer of the receiving station and back up to the application layer.Never Completely DefinedOSI was too loosely defined to become a single standard. Nevertheless, the model serves to teach communications. Compare TCP/IP, the global standard, with the OSI model (see OSI model and TCP/IP).