The Collaborative International Dictionary of English v.0.48:

Patera \Pat"e*ra\, n.; pl. Pater[ae](?). [L., fr. patere to
   lie open.]
   1. A saucerlike vessel of earthenware or metal, used by the
      Greeks and Romans in libations and sacrifices.
      [1913 Webster]

   2. (Arch.) A circular ornament, resembling a dish, often
      worked in relief on friezes, and the like.
      [1913 Webster]