calculous - n.
(pl. calculuses or calculi)
1 Math. a a particular method of calculation or reasoning (calculus of probabilities). b the infinitesimal calculuses of integration or differentiation (see integral calculusdifferential calculus).
2 Med. a stone or concretion of minerals formed within the body.

calculous adj. (in sense 2). [L, = small stone used in reckoning on an abacus]