Owen Duffy wrote:

...
I suggest that a reasonable definition of a balun is any device that
facilitates or assists transition from unbalanced to balanced mode of
operation. That definition permits a wide range of devices that may have
characteristics suited to specific applications.
...
Owen

Baluns and Ununs use a transmission line mode to accomplish their tasks
.... anything else is just a RF xfrmr ...

However, there are the 180 degree hybrid baluns ...

Regards,
JS