One of the two Grothendieck standard conjectures. See Kleiman in Motives.
http://mathoverflow.net/questions/52975/if-the-numerical-equivalence-of-cycles-coincides-with-the-homological-one-does-t
http://mathoverflow.net/questions/27858/hodge-standard-conjecture-in-positive-characteristic
nLab page on Hodge standard conjecture