Lambda the Ultimate

I realize this may be a dumb question... having had some Abstract/Modern Algebra classes, I understand what an Algebra is, but I have never really understood what defines a "Calculus". What's the common thread between Lambda Calculus, Pi-Calculus, and traditional "Calculus"(and all the other calculi...) that merits them being called a calculus?

I realize this may be an obvious thing, but my tendency is to assume that there is some formal definition of Calculus out there which is being used which I am just unaware of. Again, if I'm just being dumb, someone email me and put me out of my misery :)