a flat (usually rectangular) container for a letter, thin package, etc.
a natural covering (as by a fluid)
a curve that is tangent to each of a family of curves
a framework of wood or metal that contains a glass windowpane and is built into a wall or roof to admit light or air
a transparent opening in a vehicle that allow vision out of the sides or back; usually is capable of being opened
a transparent panel (as of an envelope) inserted in an otherwise opaque material
