Etymology of Denmark

The etymology of the name Denmark (Danish: Danmark), and especially the relationship between Danes and Denmark and the unifying of Denmark as a single kingdom, is a subject which attracts some debate. In Old Norse, the country was called Danmǫrk, referring to the Danish March, viz. the marches of the Danes. The Latin name is Dania.

According to popular legend, however, the name Denmark, refers to the mythological King Dan. There are also a number of references to various Dani people in Scandinavia or other places in Europe in Ancient Greece and Roman accounts (like Ptolemy, Jordanes, and Gregory of Tours), as well as some medieval literature (like Adam of Bremen, Beowulf, Widsith and Poetic Edda).

Most handbooks derive the first part of the word, and the name of the people, from a word meaning "flat land", related to German Tenne "threshing floor", English den "low ground", Sanskrit dhánus- (धनुस्; "desert"). The -mark is believed to mean woodland or borderland (see marches), with probable references to the border forests in south Schleswig, maybe similar to Finnmark, Telemark, or Dithmarschen.

...
Wikipedia