What is the correct term to describe a person from Mongolia; Mongol or Mongolian?

http://en.wiktionary.org/wiki/Mongol
seems to use both - Mongol and Mongolian.

From the link you will find that Mongolian refers to a person from the country, whereas Mongol is an ethnic grouping (who might come from several different countries.)
http://en.wiktionary.org/wiki/Mongol

A narrow definition includes the Mongols proper, which can be roughly divided into eastern and western Mongols. In a wider sense, the Mongol people includes all people who speak a Mongolic language, such as the Kalmyks of eastern Europe.

https://en.wikipedia.org/wiki/Mongol

The Mongolian language (Mongγol kele, Cyrillic: Монгол хэл, Mongol khel) is the best-known member of the Mongolic language family. It is the language of most residents of Mongolia and of many of the Mongolian residents of Inner Mongolia, totalling about 5.7 million speakers.

https://en.wikipedia.org/wiki/Mongolian_language

Mongol -- noun

1. a member of a pastoral people now living chiefly in Mongolia.
2. a person having Mongoloid characteristics.
3. any Mongolian language.
4. (often lowercase) Pathology. (no longer in technical use) a person affected with Down syndrome; Mongoloid.
–adjective 5. Mongolian.

Mon·go·li·an -- Of or relating to Mongolia, the Mongols, or their language or culture.
also mongolian Offensive Of or relating to Down syndrome.
n.
A native or inhabitant of Mongolia.
A member of the Mongol people.
Anthropology A member of the Mongoloid racial division.

A subfamily of the Altaic language family, Mongolian and Kalmyk being the most important members.
Any of the various spoken and written dialects and languages of the Mongols living in Mongolia and China. Also called Mongol.


http://dictionary.reference.com/browse/mongol