Difficult or convoluted.
Folded longitudinally (as in the wings of certain insects).
Single, simple; not complex.
Unidirectional.
A monomorphemic word, one without affixes.
An analogue in a space of arbitrary dimensionality of the triangle or tetrahedron; the convex hull of n+1 linearly independent points in n-dimensional space; in other words, the triangle, tetrahedron etc., generalized to an arbitrary number of dimensions.