Also called counterform.

A counter is the negative shape inside a glyph with an enclosed form, either entirely closed (such as b) or partially open (such as n).

This term comes from the counterpunch. Punchcutters used counterpunches to depress the white spaces inside letterforms while making the master punches for metal-type sorts.
Concretely, they would strike the counterpunch into a larger steel punch. Afterward, the punchcutter filed down the outside of the punch, creating the letterform’s exterior contour. Punchcutters could reuse a counterpunch on any related letterforms in the typeface, keeping a consistent look throughout an entire font.

TYPE DESIGN

In digital type design for Latin scripts, it is important to keep this consistency as well, with various techniques, such as putting the component of a reference letter in the background or copy-pasting the contours of the counter, or making sure that they visually look consistent by placing them next to each other while designing the shapes.