What is NATO Strategic Concept?

The NATO Strategic Concept documents are a series of visionary instruments or White papers which serve to outline NATO's "enduring purpose and nature, its fundamental security tasks, and the challenges and opportunities it faces in a changing security environment. It also specifies the elements of the Alliance’s approach to security and provides guidelines for its political and military adaptation." The document has been re-written on a period exceeding ten years. The series has been characterized by at least one observer as NATO's "most central document, outlining its purpose and tasks."