The Collaborative International Dictionary of English v.0.48:

Folium \Fo"li*um\, n.; pl. E. Foliums, L. Folia. [L., a
   leaf.]
   1. A leaf, esp. a thin leaf or plate.
      [1913 Webster]

   2. (Geom.) A curve of the third order, consisting of two
      infinite branches, which have a common asymptote. The
      curve has a double point, and a leaf-shaped loop; whence
      the name. Its equation is x^3 + y^3 = axy.
      [1913 Webster] Folk

WordNet (r) 3.0 (2006):

folium
    n 1: a thin layer or stratum of (especially metamorphic) rock