I'm tempted to offer a really simple definition: "Salon music is music
that isn't concert music."  Small scale, not too attention-grabbing,
not too hard to play, as other contributors have said; it was played to
fill in those long Sundays at home before the gramophone, radio or TV.

In the grander English houses of the era, the salon would be the name
of the room for such entertainments (the drawing room was where you
went after dinner, being short for 'withdrawing room', and a good
hostess would want separate rooms for each activity!).  As always in
English, the use of a French term shows the implied refinement of the
activities therein.

To me, it's the domestic music of the era between the advent of the
domestic piano and ragtime music.  Ragtime and onwards were rather too
involving to be salon music (and were widely criticised for having too
strong an effect on the listener), although I suppose a suitably
underwhelming performance could salonise anything.

Today it provides a nicely dismissive term for music that the speaker
thinks is bland, boring, safe, unimaginative...  But as we know,
insults normally say more about the person issuing them than the
thing they're insulting!

There's lots of this music on player and reproducing piano rolls, and
perhaps in higher proportion here in the UK due to the later and less
comprehensive adoption of jazz and its derivatives.  Well played, as
many reproducing piano rolls are, MMDers know these lighter pieces can
actually be rather enjoyable. I wonder just how many other groups ever
hear any of this repertoire these days?

Julian Dyer