The stamps issued by Nagorno-Karabakh aren't recognized on an international level, either by UPU nor by PostEurop. But those stamps are listed in most stamps catalogues. Nagorno-Karabakh or Mountainous Karabagh is a landlocked region in the South Caucasus and is recognized by the United Nations as part of Azerbaijan.
The stamp, issued in sheets of 8 stamps and miniaturesheets shows the monument "We Are Our Mountains" in the north of Stepanakert.
(it does not correspond with the real view, background unknown).