A baktun was a unit of time used by the now-extinct Tkon Empire.
The Ages of Tkon, which marked eras in their history, were divided into baktun. According to the Portal, there was seven baktun of time separating the Age of Makto from the Age of Fendor. (TNG: "The Last Outpost")
The Mayan calendar had a unit of the same name, Baktun.