A nef divisor is a real Cartier divisor 
on a projective algebraic
variety 
whose intersection number 
is nonnegative for
every integral algebraic curve 
. The term abbreviates numerically effective (Kollár
and Mori 1998). The definition includes rational and integral Cartier
divisors.
A nef divisor need not be ample or have a positive multiple generated by global sections. For example, the zero Cartier divisor is nef. In the minimal model program, nefness of the adjoint divisor is the defining endpoint for a minimal model.
