Assuming it's not the negative carrier (usually made of metal and glass), the term you're looking for is a "mask". Used to mask off a small area of a big negative, or place a small negative in a much larger carrier while blocking (flare) light from going around the little neg.

Probably best used with a glass carrier so that the mask can press the neg against the glass. If the carrier is glassless, you will want a pair of masks in order that the neg be fully supported on all edges.