Let's suggest we have a binary relation $R$ such that: $$aRb$$
I am wondering what is the proper term for the individual elements $a$ and $b$ in the context of relation $R$.
Coming from the IT world, I would perhaps refer to $a$ as the key and $b$ as the value. But this does not feel like the proper terminology in mathematics and, to my surprise, I was unable to find the answer at my usual go-to places for such matter.
Any suggestion is highly appreciated.