An interactive stream is one on which it makes sense to perform interactive querying.
The precise meaning of an interactive stream is implementation-defined, and may depend on the underlying operating system. Some examples of the things that an implementation might choose to use as identifying characteristics of an interactive stream include:
The general intent of having some streams be classified as interactive streams is to allow them to be distinguished from streams containing batch (or background or command-file) input. Output to batch streams is typically discarded or saved for later viewing, so interactive queries to such streams might not have the expected effect.
Terminal I/O might or might not be an interactive stream.