 Fontaine's period rings

Fontaine's period rings are a concept in the field of arithmetic geometry and number theory, specifically related to p-adic Hodge theory. They were introduced by Pierre Fontaine in the context of understanding the relationships between different types of cohomology theories, particularly for p-adic representations of the absolute Galois group of a p-adic field. More concretely, Fontaine's period rings provide a framework for studying p-adic Galois representations and their associated periods.