Sucht means a disease or illness -- cognate, naturally, to English "sick". So Sehnsucht, which is usually translated as "longing", is a stronger word than that -- it means something like "sick with longing."
Niche is not a German word. It is derived from the French word niche (recess), which in turn comes from the Latin nidus, meaning nest.The word means:a shallow recess, especially one in a wall to...