Definition of National Park by IUCN

National Park: (Category II)

Protected area managed mainly for ecosystem protection and recreation.

Natural area of land and/or sea, designated to
  1. protect the ecological integrity of one or more ecosystems for present and future generations,
  2. exclude exploitation or occupation inimical to the purposes of designation of the area and
  3. provide a foundation for spiritual, scientific, educational, recreational and visitor opportunities, all of which must be environmentally and culturally compatible.

IUCN, 1994. Guidelines for Protected Areas Management Categories. IUCN, Cambridge, UK and Gland, Switzerland. 261pp.