Younger Dryas
A period 12.9 to 11.6 kya, during the deglaciation, characterised by a temporary return to colder conditions in many locations, especially around the North Atlantic.
Definition courtesy of IPCC AR4.
All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.