From AMS Glossary
An interval of higher summer temperatures (2°–4°C higher) that followed the retreat of the last Pleistocene glaciers in North America and Europe, varying in age with location but often between approximately 5000 and 7000 years ago.
The term is sometimes applied to warm and wet conditions that occurred during this time in some regions, although the warmth was probably accompanied by dryness in other regions.
Kutzbach, J. E. 1992. Modeling earth system changes of the past. Modeling the Earth System. D. Ojima, Ed., UCAR/ Office for Interdisciplinary Earth Studies, Boulder, CO.