A ghost is an anerythristic who is also hypomelanistic (Hypo type A).
There is some crossover between very light anerys and very dark ghosts, but the genetics defines the snake. If it's homozygous (carries both genes for) both anery A and hypo A, it's a ghost by definition.
I'll post a few pics when I get home of hatchlings and adults if someone hasn't covered it already.