A thin layer or stratum of (especially metamorphic) rock

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Folium \Fo"li*um\, n.; pl. E. Foliums , L. Folia . [L., a leaf.] A leaf, esp. a thin leaf or plate. (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; ...

Wikipedia Word definitions in Wikipedia
Folium (, "leaf") may refer to a leaf of a book: see recto and verso Folium of Descartes , an algebraic curve Folium (spider) , a marking on the abdomen of a spider

WordNet Word definitions in WordNet
n. a thin layer or stratum of (especially metamorphic) rock [also: folia (pl)]

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
see folio .

Wiktionary Word definitions in Wiktionary
n. 1 A leaf, especially a thin leaf or plate. 2 (context geometry English) A curve of the third order, consisting of two infinite branches having a common asymptote. The curve has a double point, and a leaf-shaped loop.