Even though the entire working, or even just the ball can be referred to as a "detent" (even though this is the first time I've heard someone use that term for the pill), as you probably figured by now most people consider the detent to be the notch only. Whereas the other working part of the detent (beside the spring) is commonly known as the detent ball, or the "pill" they're talking about.
With the detent being the notch in the rod and the pill being the ball that slips into it, it's easier to describe and differentiate the component parts.
At least that's how I figure it. Since I wasn't actually there when they first designed it and made up words to describe it, I just go by how I lernt it!%)
Paul