A specialized pipe or hollow passage placed inside a casting mold that allows cooling water to flow through it while the molten metal solidifies. This acts as an extreme internal chill, rapidly freezing the surrounding metal to refine the grain structure, prevent shrinkage voids in heavy sections, or form a permanent cooling jacket in the final part.
Heard on the job
We need a water core in the center of that massive bronze bushing or it will take three days to solidify and shrink into a sponge.