A set of network protocol layers that work together. The OSI Reference Model that defines seven protocol layers is often called a stack, as is the set of TCP/IP protocols that define communication over the internet.
The term stack also refers to the actual software that processes the protocols. So, for example, programmers sometimes talk about loading a stack, which means to load the software required to use a specific set of protocols. Another common phrase is binding a stack, which refers to linking a set of network protocols to a network interface card (NIC). Every NIC must have at least one stack bound to it.