Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.

Source : WordNet®

folium
     n : a thin layer or stratum of (especially metamorphic) rock
     [also: {folia} (pl)]