What is a baktun?

A baktun is 20 katun cycles of the ancient Maya Long Count Calendar. It contains 144,000 days, equal to 394.26 tropical years. The Classic period of Maya civilization occurred during the 8th and 9th baktuns of the current calendrical cycle. The current (13th) baktun will end, or be completed, on 13.0.0.0.0 (December 21, 2012 using the GMT correlation). This also marks the beginning of the 14th baktun, as such a term is usually used among Mayanists.