What is meant by a mixed characteristic field?

In the context where Scolze is using this term, such a fields comes with a valuation. The corresponding valuation ring has a residue field (this is the quotient by its unique maximal ideal). If the characteristic of this residue field is different from that one of the initial field, then one says that the initial field is of mixed characteristic. Note that this can only happen if the initial field has characteristic 0.

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