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What is the difference between calculus and calculi?

Both of these words are used to identify formal languages in computer science (e.g. π-calculus, λ-calculus, process calculi).

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Calculus is singular.

Calculi is plural.

"Predicate calculus is the same as first order logic"

"Process calculi are used to analyze temporal properties of systems"

This is the direct borrowing from Latin of their pluralization of 2nd declension '-us' words, in the same class as 'alumnus (sg), alumni (pl)'. Not all foreign language borrowings maintain the original language's morphological rules, but this one does.

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To explain further, from Wikipedia, "In computer science, the process calculi (or process algebras) are a diverse family of related approaches to formally modelling concurrent systems. ... Leading examples of process calculi include CSP, CCS, ACP, and LOTOS. More recent additions to the family include the π-calculus, the ambient calculus, PEPA and the fusion calculus." So process calculi refers to a family of things, some of which are named a "calculus". – Peter Shor Sep 23 '11 at 12:27
The plural calculuses is also accepted. – GEdgar Sep 23 '11 at 13:53
this is a site about english and not classics, right? – Jodrell Sep 23 '11 at 16:57
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At arxiv.org, Google says that calculi is more frequent than calculuses by a factor of 100; calculuses may be accepted by some dictionaries, but it is barely used in technical contexts. – Peter Shor Sep 23 '11 at 18:05

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