Counter

Counters do the work of telling letters apart. An e, an o and a c differ mostly in how their inner space is closed off, and when that space gets small the three start to converge. That is exactly what happens as type gets smaller, as it gets heavier, or as it is printed on something absorbent, which is why a face intended for small sizes is drawn with counters more open than they need to look at display size.

The related measurement is the aperture: how far the counter is left open. A c with its terminals curled tightly toward each other has a closed aperture and turns into an o at a distance; one with the terminals swung apart stays a c. Faces drawn for signage and for interfaces almost always open their apertures for this reason.