terminology: what is meant if someone writes “calculus of ..”?

This question might be a little soft as it does not have a definite answer, so I hope I do not break the conventions of this forum by posting it here.

I have now come across the term "calculus of operators" several times in various books, as well as the "calculus of several variables", "calculus of variations", calculus of residues", "calculus of pseudodifferential operators", and so on.

Each time I don't really understand what is meant by this - my guess is it involves an algebraic system that contains operations broadly to be understood as differentiation and integration - but I am not sure at all, in fact I don't think it fits the bill.

I would be really grateful if I could get some better explanation than mine for the term "calculus of [something]". Many thanks !

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"calculus of [...]" means "systematic computational procedure related to [...]". For example, residue calculus is the ideas related to using residues to compute integrals. A functional calculus (in functional analysis) is a process of making sense of non-polynomial functions (continuous, holomorphic, measurable) of linear operators. Historically, differential and integral calculus were such a substantial advance that the undecorated label "calculus" was understood to refer to them. The effect is that you think any other calculus is somehow connected to it, but that's wrong. – KCd Apr 2 '12 at 0:24
Rules (okay, system) of calculation. That's why you see process calculus, lambda calculus. – user2468 Apr 2 '12 at 1:39
Note that calculus means small stone. So if you have calculi in the kidneys, or the gall bladder, it can be very painful. It all comes from the time that calculi were used on counting boards, or variants of the abacus. By the way, the abacus was known and used very early in the Mediterranean world, and then disappeared. – André Nicolas Apr 2 '12 at 5:19
Dentists who find calculus in your teeth need to remove it. To them it still has the original meaning: a small stone. – Michael Hardy Apr 2 '12 at 17:23
@André The abacus did not disappear from Europe. It was widely used in various forms up until modern times. The English department of the Exchequer is named for the checkered cloth that was used to turn the counting table ("counter") into an abacus. See this picture for an example of a Medieval "rechentisch" = "counting table", on which small tokens ("counters") were moved about in a manner identical to the movements of beads on an abacus. – MJD May 30 '12 at 16:42