A megagon is a polygon with 1 million sides (from mega-).

A regular megagon has an interior angle of 179.99964°.

The perimeter of a regular megagon inscribed in the unit circle is:

which is very close to 2π. In fact, for a circle the size of the Earth, with a circumference of 40,000 kilometres, the difference between the perimeter of the megagon and the circumference of the circle comes to less than 1/16 millimetre.

As with René Descartes' example of the chiliagon, the million-sided polygon has been used as an illustration of a well-defined concept that cannot be visualised.

The megagon is also used as an illustration of the convergence of regular polygons to a circle.

Date Masamune (伊達 政宗^?, September 5, 1567 – June 27, 1636) was a regional strongman of Japan's Azuchi-Momoyama period through early Edo period. Heir to a long line of powerful daimyo in the Tōhoku region, he went on to found the modern-day city of Sendai. An outstanding tactician, he was made all the more iconic for his missing eye, and Date was often called dokuganryū (独眼竜), or the "one-eyed dragon".

As a legendary warrior and leader, Masamune is a character in a number of Japanese period dramas. He was played by Ken Watanabe in the popular 1987 NHK series Dokuganryu Masamune.

Date Masamune was the eldest son of Date Terumune, born in Yonezawa Castle (in modern Yamagata Prefecture). At the age of 14 in 1581 Masamune led his first campaign, helping his father fight the Sōma family. In 1584, at the age of 17, Masamune succeeded his father, Terumune, who chose to retire from his position as daimyo. The Date family was founded in the early Kamakura period by Isa Tomomune, who originally came from the Isa district of Hitachi Province (now Ibaraki Prefecture). The family took its name from the Date district (now Fukushima Prefecture) of Mutsu Province, which had been awarded to Isa Tomomune by Minamoto no Yoritomo, the first Kamakura shogun, for his assistance in the Genpei War (1180–85) and in Minamoto no Yoritomo’s struggle for power with his brother, Minamoto no Yoshitsune. Masamune's army was recognized by its black armor and golden headgear.