A Tiffany mount or Tiffany setting is a set of prongs (called a claw, crown, or coronet) that hold a gemstone and attach it to a plain band ring. It is the most common setting for a diamond solitaire engagement ring. It is named after Tiffany & Co., which popularized it.
It is inexpensive and easy to re-use, so most jewelers will accept returns of this setting and re-set the diamond in a different ring more to the bride-to-be's liking. (This is most often done just before getting married, to move the diamond into a wedding set of wedding band and engagement ring that are matched and often locked together.)
The Tiffany mount also shows off the diamond and may maximize its radiance. Quality settings do not snag on clothing, as the prongs are firmly attached to the diamond and well finished.