Blocking anticyclone
An anticyclone that remains nearly stationary for a week or more at middle to high latitudes, so that it blocks the normal eastward progression of high- and low-pressure systems.
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.