Wikipedia defines counter then gives examples.
Then it proceeds to give examples of open counter without having given any definition of it:
In typography, a counter or aperture is the area of typeface anatomy that is entirely or partially enclosed by a letter form or a symbol (the counter-space/the hole of). Letters containing closed counters include A, B, D, O, P, Q, R, a, b, d, e, g, o, p, and q. Letters containing open counters include c, f, h, i, s etc.
So, what is the proper definition of "open counter"?
To illustrate, could you give a handful examples of characters that contain no open counters?
Is it synonymous with "aperture"? 'Fonts and Encodings', O'Reilly 2007. Pg. 4 says:
The counter, which is the inner part of a letter, for example, the space inside an 'o' and 'O', a 'D', etc. The counter of an 'e' is commonly called an eye. When the letter is open at one end, as is the case with 'n' we speak instead of an aperture.
I have drawn my interpretation of open counter in green below, is it correct?