Why does "counter" mean the area of a letter entirely, or partially, enclosed by a letter form or a symbol?
Etymonline's entries for the homonyms "counter" don't semantically appertain to its meaning in typography. How does "counter" in typography relate to the common lay English 2021 meanings of "counter"? Solely Wrzlprmft♦'s answer distinguishes "counter" from "aperture".
Why did the typography industry choose "counter" to signify this concept?
Counters are the negative spaces inside letter forms. In the images below you can see all counters in capital and lowercase letters with closed letter forms, with the counters in blue and green.
Closed counters are in medium blue, while open counters are in light blue.
Closed counters are in medium green, while open counters are in light green.