From 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
From WordNet (r) 3.0 (2006) :
folium
n 1: a thin layer or stratum of (especially metamorphic) rock