An urn is a vase, ordinarily covered, that usually has a narrowed neck above a footed pedestal. "Knife urns" placed on pedestals flanking a dining-room sideboard were an English innovation for high-style dining rooms of the late 1760s. They went out of fashion in the following decade, in favour of knife boxes that were placed on the sideboard.
In Classical terms, an urn is a large decorative covered container of wood, metal, pottery, etc. In furniture, it was a large wooden vase-like container which was usually set on a pedestal on either side of a side table. This was the characteristic of Adam designs and also of Hepplewhite's work. Urns were also used as decorative turnings at the cross points of stretchers in 16th and 17th century furniture designs. The urn and the vase were often set on the central pedestal in a "broken" or "swan's" neck pediment.
Read more about Urn: Cremation Urns, Other Urns