From the American Museum of Natural History: The Lives of Birds, Birds of the World and Their Behaviour

Biblio’s Book Conditions

• As New
  The book is pristine and free of any defects, in the same condition as when it was first newly published.
• Fine (F)
  A book in fine condition exhibits no flaws. A fine condition book closely approaches As New condition, but may lack the crispness of an uncirculated, unopened volume.
• Near Fine (NrFine or NF)
  Almost perfect, but not quite fine. Any defect outside of shelf-wear should be noted.
• Very Good (VG)
  A used book that does show some small signs of wear - but no tears - on either binding or paper. Very good items should not have writing or highlighting.
• Good (G or Gd.)
  The average used and worn book that has all pages or leaves present. ‘Good’ items often include writing and highlighting and may be ex-library. Any defects should be noted. The oft-repeated aphorism in the book collecting world is “good isn’t very good.”
• Fair
  It is best to assume that a “fair” book is in rough shape but still readable.
• Poor (P)
  A book with significant wear and faults. A poor condition book can still make a good reading copy but is generally not collectible unless the item is very scarce. Any missing pages must be specifically noted.