Part of a data flow, also called a Stream Operator. The specification of a data flow is called a physical query plan. The actual data flow is called a stream graph, which is a chain of stream nodes, each of which is a small software machine that applies some relational operation to its input data stream(s) to produce is output data stream(s).

For more information see Background Concepts: Stream-graphs, Nodes, and s-Server in the Administration Guide.