Good describes the average used and worn book that has all pages or leaves present. Any defects must be noted. (as defined by AB Bookman's Weekly).
The oft-repeated aphorism in the book collecting world is "good isn't good." However, this can be a little misleading: while generally not considered an acceptable condition for collectors (except in cases of very scare material), a good condition book usually suffices as a reading copy.
Good is most often abbreviated in listings as G or Gd.