[mruts]

I always thought it related to continuous variables and the highly useful concept of the infinitesimal. But maybe that definition works as well.

[User23]

I thought it was just yet another historical and linguistic oddity in English.

[mruts]

Calculus means "stone" in Latin. I suspect the relationship between stones and counting is why we use it so much (though I'm not an expert on how/why Romans used stones).