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As you should infer from Brian & Samuel's posts, there is no strict universally-accepted definition for "impedance balanced." For that matter, most of the quote-unquote "strict" definitions for any of those terms are not universally accepted, and vice-versa.
Note that the strict definition of "balanced" makes no mention of the symmetry of the signal on the + and - conductors, it only stipulates that the impedance of those conductors with respect to ground (or to one another) is identical.
This definition seems really bizarre to folks who grew up around audio but didn't study electronics in depth. Like me. And, perhaps fortuitously, like the guys in the marketing departments of audio companies who proudly tout their stuff as featuring "impedance balanced" i/o.
While the IEC insists that *any* balanced circuit be "impedance balanced", the marketing guys use this term to differentiate gear which does *not* have symmetrical signals on its + and - conductors from gear which does
...I think.
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