Grizzle is a term which implies an intermixture of black and white.
Grizzled hair is hair which has both color and white on the same shaft.
Another term we could use for it is salt and pepper. Grizzle in pigeons means the same thing, a mix of white and color on the same feather, i.e., the barbules of each feather are white and colored in a seemingly random mixture. It is this coloration which pigeon breeders usually refer to when they mention grizzle. Grizzle has been found to be partially dominant to wild type. The one breed most people probably think of when they mention grizzle is the Dragoon.