Quartic operator definition

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What is a quartic operator? I googled it but found only some articles which use that term whitout giving a definition (I found that term while studying 2D Ising model, and the use of some transformation (Rotation, Jorndan-Wigner, Fourier) to get the free energy. My quaestion arises because in the notes on which I am studying I found that phrase: "whitout performing the rotation transformation we would have obtained a quartic operator instead of a bilinear one". Could you give me a brief definition of quartic operator or suggest a good reference in which I can found at least a definition?