Dryas is the name for the short cold periods at the end of or just after the last ice age and during which the climate changed suddenly. Two Dryas periods are generally accepted, but some people consider that there were three. They occurred at the end of the Pleistocene epoch, therefore just before the beginning of the Holocene (the current epoch).
The Older Dryas marked the beginning of the thaw, over 14,000 years ago, and therefore the end of the last ice age. This period is thought to have lasted three centuries. A short Dryas episode could have occurred around 12,000 years ago.
The Younger Dryas occurred about 12,000 years ago. While the planet's climate was warming over a long time-scale, this period marked a very sudden cooling 1300 years ago.
The name Dryas comes from a flower found in the Alps and tundra. (Mountain avens, Latin name dryas).