Is it ensure, insure, or assure?
folium fo·li·um (fō'lē-əm)n. pl. fo·li·a (-lē-ə) A broad, thin, leaflike structure, as of the cerebellar cortex.
A thin, leaflike layer or stratum occurring especially in metamorphic rock.
A plane cubic curve having a single loop, a node, and two ends asymptotic to the same line. Also called folium of Descartes.