The definition has been found on the Wikipedia website

A National Park is a park in use for conservation purposes. Often it is a reserve of natural, semi-natural, or developed land that a sovereign state declares or owns.

Although individual nations designate their national parks differently, there is a common idea: the conservation of wild nature for posterity and as a symbol of national pride.

Furthermore, an international organisation, the International Union for Conservation of Nature (IUCN), and its World Commission on Protected Areas have defined “National Park” as a Category II type of protected area. While ideas for this type of national park had been suggested previously, the United States established the first one, Yellowstone National Park, in 1872.

In 1969 the IUCN declared a national park to be a relatively large area with the following defining characteristics:

• One or several ecosystems not materially altered by human exploitation and occupation, where plant and animal species, geomorphological sites and habitats are of particular scientific, educational, and recreational interest or which contain a natural landscape of great beauty;
• The highest competent authority of the country has taken steps to prevent or eliminate exploitation or occupation as soon as possible in the whole area and to effectively enforce the respect of ecological, geomorphological, or aesthetic features which have led to its establishment; and
• Under special conditions, visitors are allowed to enter for inspirational, educative, cultural, and recreative purposes.

In 1971 these criteria were further expanded, leading to more transparent and defined benchmarks to evaluate a national park.

These include:

• Minimum size of 1,000 hectares within zones in which protection of nature takes precedence;
• Statutory legal protection;
• Budget and staff sufficient to provide sufficient adequate protection
  
• Prohibition of exploitation of natural resources (including the development of dams) qualified by such activities as sport, fishing, the need for management, facilities, etc.

While national parks are generally understood to be administered by federal governments (hence the name), in Australia, national parks are run by state governments and predate the Federation of Australia; similarly, national parks in the Netherlands are administered by the provinces.

In many countries, including Indonesia, the Netherlands, and the United Kingdom, national parks do not adhere to the IUCN definition. In contrast, some areas adhering to the IUCN definition are not designated national parks.