Counterpart means that something goes with or is somewhat equivalent to something else. So for your example sentence, AD (Anno Domini) has the same use or can be paired with BC (Before Christ). AD and BC don’t have the same meaning, but they are grouped together because they are two ways of expressing a period of time. Another example: “The male counterpart to a hen is a rooster.” So a hen and rooster go together, even though they aren’t exactly the same. I hope this helps!

Hello! The word "counterpart" means a person or thing that corresponds to or has the same function as another person or thing in a different place or situation. The terms anno Domini (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendars. So the writer is talking about two acronyms that are used for the same thing - they are therefore connected and that is why he calls the one a counterpart of the other. I hope that helps!

:)