A concave mold is called a Cavity and a convex mold is called a Core. The sizes of cavity and core are different because of the thickness of the part. A part will be made by pouring plastics and etc. in the opening which exists between the two molds when putting them together.
Cavity and core are types of molds and the names of molds which can process a part in the opening which exists between concave mold and a convex mold when they are closed. Cavity is a concave mold and Core is convex mold.
However, if both molds are concave (like forging molding), since you cannot call them like the above, the mold which can move is called "Movable mold" and the mold which cannot move is "Fixed mold".
The side where the cavity and core (or movable mold and fixed mold) meet is called PL (Parting Line) side or parting side.