> William Oughtred, in his 1631 work Clavis Mathematicae (The Key of Mathematics), used the notation “π/δ” where π is the circumference of a circle and δ its diameter
This notation would make more sense if you described it as referring to the "perimeter" of a circle, which it seems almost certain is what William Oughtred had in mind. (Though maybe he was thinking "periphery", which is a more exact match to "circumference". I don't know where "circumference" came from; there is no conceptual difference to explain the difference in word use, and it's strange for a Latin word to be used in geometry anyway.)
This notation would make more sense if you described it as referring to the "perimeter" of a circle, which it seems almost certain is what William Oughtred had in mind. (Though maybe he was thinking "periphery", which is a more exact match to "circumference". I don't know where "circumference" came from; there is no conceptual difference to explain the difference in word use, and it's strange for a Latin word to be used in geometry anyway.)