Difference between "calculus" and "calculi"

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).

Solution 1:

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.