Riser

A riser is a reservoir of material built into the mold to prevent porosity resulting from shrinkage.

It prevents porosity by providing molten metal to the casting as it solidifies, so that porosity forms in the riser and not the casting. After solidification, the riser is removed and the part is free of porosity.
Figure 1. Example of a Riser Positioned in Relation to the Part and Pouring Cup