Nef line bundle

Summary

A line bundle on a proper variety is nef, short for numerically effective or numerically eventually free, when its degree on every curve is nonnegative. Nef line bundles are the limits of ample line bundles: on a projective variety, the nef cone is the closure of the ample cone. Nefness is a numerical condition, so it depends only on intersection numbers with curves. It is central in the minimal model program, where a minimal model is a variety with mild (terminal) singularities whose canonical divisor is nef.