In my experience it's any music that was being made the
way you make it prior to your learning to make it that way,
and developing a closed mind about how it should be done
'properly.' Or being made the way you first heard it.

I began to develop this opinion on being informed at a
seisun in New Jersey, that my playing of 'chunes' on a
melodeon which was not tuned in the keys of B/C, C/C#,
or C#/D wasn't 'traditional.' They'd never heard of how
the technique was actually developed between the wars.