Flatness is a property of morphisms between algebraic varieties, indicating that the fibers over points are 'uniform' in a certain sense. It describes how a family of varieties varies smoothly, ensuring that the structure of the fibers does not vary too wildly as one moves through the base space. Flatness is crucial for ensuring that certain geometric and algebraic properties are preserved under base change, playing an essential role in various constructions such as Chow rings and Hilbert schemes.
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