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