Clathrate
Also: methane clathrate
methane hydrate
A partly frozen slushy mix of methane gas and ice, usually found in sediments in conditions of low temperatures and high pressures
Definition based on (revised from) 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.