Nibiru (sometimes misspelled as "Niburu"), according to the pseudoscientific ideas of Zecharia Sitchin, is the name of a "twelfth planet" in the Solar System with a highly eccentric orbit that periodically comes close to the Earth, causing cataclysms. Sitchin based his claims on creative (mis)translations of ancient Sumerian texts and on no-less-creative interpretation of ancient Sumerian images.
 Planet recycling
After Nancy Lieder's Planet X fiasco in 2003, hers and Sitchin's imaginary planets became conflated and are peddled by various individuals as a cause for a 2012 apocalypse. It totally happened that time.