> Is there a standard definition for this at all?

Kinky!  It may be lost in the mists of time :)

> The question which started my search is that our
> requirements ask for a transparent end-to-end
> asynchronous data service. Would transporting
> a break condition be included? and transporting
> strange things like fractional data bits?

It COULD be done that way, basically providing a "wire" between points, but
you'd always have some minimum timeslice which would represent a maximum
bitrate.
I assume you're doing some kind of digital link here?

The break signal is, as far as I can tell, a hack. It induces a framing
error at the other end, with a byte value of 00, but there is no firm
definition of how long a break should be.  I've seen "specs" for break of
100mS, 11 bit times, several seconds.....

I would think though, that the fact that they call it out as async would
indicate that there are no fractional bits, and that there would be a
defined max bitrate, and a defined format.