Tim Mahon wrote:

>Don't know -- but I will add some confusion.  It's not only used in Opus
>numbers but also in work titles on occasion.  For example, Villa-Lobos'
>"Two Choros Bis for violin and cello"

The term usually implies a revision.  I am fairly certain that it literally
means "after." Since the Prokofiev second violin sonata, for example, was
also reworked by the composer for flute, the flute version might be known
as Opus X bis, so that one could tell that the music was a revision or
alteration of a pre-existing and recognized work.

Kevin