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The bispectrum is the Fourier transform of a third-order cumulant or correlation function, with the convention specified for the process or field. It measures third-order dependence among frequency triples; the primordial bispectrum is a cosmological random-field application.

Statistical homogeneity defines the primordial bispectrum by

A bispectrum is a Fourier-space third-order statistic; primordial perturbations supply the cosmological field in this instance.

The trispectrum is the Fourier transform of a fourth-order cumulant, a fourth-order counterpart of the bispectrum. It describes connected four-point correlations; the primordial trispectrum applies it to cosmological random fields.