What excatly does 'calculus' mean?

[Ella]

What is calculus in the following sentence?

He could answer questions on calculus when he was five and do algebra when he was eight.

And give a couple of sample questions you would ask on calculus, please.

[Torsten]

Any question referring to a method of mathematical computation is a question on calculus.

TOEIC listening, photographs: Boarding the schoolbus

[ChoTokusen]

Calculus is a mathematical theory concerned with change. The Russian term for Calculus is "Дифференциальное и интегральное исчисление". Calculus is divided into Differential calculus and Integral calculus which are linked through the Fundamental theorem of Calculus. It is a big step up from algebra so the sentence sounds illogical to me. As for the questions, a typical question of calculus is to minimize the area of a surface containing a fixed volume.