A dynamic variable is a module-scoped variable you can define that can thereafter be referenced by name in expressions in operators and adapters in the containing module. Dynamic variables are linked to an input or output stream in the containing module. The value of a dynamic variable is updated when a tuple is received on the linked input stream or when a tuple is emitted on the linked output stream.
The analogous feature for StreamSQL is the DECLARE statement. For both EventFlow and StreamSQL applications, the scope of a dynamic variable is the containing module.
See Using Dynamic Variables for details on this feature, and for a brief tutorial that clarifies the purpose and usefulness of dynamic variables.