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 Brain anatomy Folium (brain) Folium vermis Botany Turnsole or folium, a dyestuff Folium Phyllostachydis, processed leaves of bamboo Hydrangeae Dulcis Folium, processed leaves of hydrangea