The term c-number (or classical number) is an old nomenclature used by Paul Dirac which refers to real and complex numbers. It is used to distinguish from operators (q-numbers or quantum numbers) in quantum mechanics.
Although c-numbers are commuting, the term anti-commuting c-number is also used to refer to a type of anti-commuting numbers that are mathematically described by Grassmann numbers. The term is also used to refer solely to "commuting numbers" in at least one major textbook.
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