Thirteen

The cardinal number occurring after twelve and before fourteen, equivalent to the sum of six and seven; one more than twelve, or seven less than twenty represented in Roman numerals as XIII and in Arabic numerals as 13.

Trivia
Thirteen's numeral means:


 * 7 in Quaternary.
 * 9 in Senary.
 * 11 in Octal.
 * 15 in Duodecimal.
 * 19 in Hexadecimal

In the hexadecimal system, the numeral for 13 is D.

Thirteen is considered as an unlucky/evil number, so much so that there is no thirteenth floor/row in airplanes or hotels