An adapter is a processing unit which (in most cases) has an input connection and an output connection.

Data flows through an adapter in the shape of granules.

The adapter accepts granules through its input connection, and emits granules through its output connection.

Some adapters have additional output connections - e.g. an input splitter is an adapter which connects to many output adapters.

Some adapters might not produce any output, and act as a sink. They might be collating their input data into a common data pool, where it can be picked up later by another adapter, or they might be sending the data they receive it to the outside world, and have no need to pass it on to an adapter further in the chain.