From AMS Glossary
The period from approximately 10 800 to 9600 BC when climate in the region around Greenland cooled by 5°–7°C within a few decades and recovered with similar rapidity at the end of the period; named for the expansion of the geographic range of the arctic herb Dryas octopetala.
Evidence is accumulating for a wider, perhaps global, extent, but the Younger Dryas was most strongly felt around the North Atlantic Ocean. Changes in the thermohaline circulation of the oceans are widely believed to be implicated in the onset and ending of the Younger Dryas cooling. These were in turn associated with a switch to the St. Lawrence River as the major route for meltwater reaching the North Atlantic from the retreating continental ice cap. Other similarly rapid oscillations in climate are recorded through much of the last glacial in ice cores from Greenland and in marine sediment cores from the North Atlantic (Dansgaard–Oeschger events). The period since the Younger Dryas (the Holocene) lacks such extreme excursions.