The tag calculus has been created. The 9 challenges linked in the question have been retagged to add this.
At least for starters, I've added the following information to the tag wiki.
Use this tag for challenges involving integration or differentiation of functions.
Calculus, not to be confused with lambda calculus, is the mathematical study of change. The main branches are differential calculus, which involves taking derivatives of functions, and integral calculus, which involves integrating functions.
This tag should be used for any challenge which requires numerically or symbolically differentiating or integrating functions.
We should be vigilant in making sure that challenge authors do not confuse calculus with lambda-calculus, as they are entirely separate.