[_delirium]

A calculus is a system for calculation, so traditionally a specific system for calculation is named as the X calculus: the integral calculus, the pi calculus, the lambda calculus, the situation calculus, etc.

As a loose analogy, think of how specific instances of the general idea of systems are named: the court system, the cooling system, the moderation system, etc. Some uses are a bit archaic though, e.g. people now usually refer to integral calculus as a standalone name, without the definite article. I think we're somewhere in between with (the) lambda calculus; you can find papers that use "the" and others that don't.