A feature of Unix System V that provides a standard way of dynamically building and passing messages up and down a protocol stack. STREAMS passes messages from the application "downstream" through the STREAMS modules to the network driver at the end of the stack. Messages are passed "upstream" from the driver to the application. A STREAMS module would be a transport layer protocol such as TCP and SPX or a network layer protocol such as IP and IPX.

STREAMS modules can be dynamically changed (pushed and popped) at runtime, allowing the stack to be used for multiple protocols. Two important STREAMS components are the TLI and LSL interfaces, which provide common languages to the transport and data link layers. See TLI, LSL, ODI and OSI.