Schematic overview of a thrust system. The hanging wall block is (when it has reasonable proportions) called a nappe. If an erosional hole is created in the nappe that is called a window. A klippe is a solitary outcrop of the nappe in the middle of autochthonous material.
A tectonicwindow (or Fenster (lit. "window" in German)) is a geologic structure formed by erosion or normal faulting on a thrust system. In such a system the rock mass (hanging wall block) that has been transported by movement along the thrust is called a nappe. When erosion or normal faulting produces a hole in the nappe where the underlying autochthonous (i.e. un-transported) rocks crop out this is called a window.
Windows can be almost any size, from a couple of metres to hundreds of kilometres.