noun

• (dated, countable) Calculation; computation.
• (countable, mathematics) Any formal system in which symbolic expressions are manipulated according to fixed rules.
• (uncountable, often definite, the calculus) Differential calculus and integral calculus considered as a single subject; analysis.
• (countable, medicine) A stony concretion that forms in a bodily organ.
• (uncountable, dentistry) Deposits of calcium phosphate salts on teeth.
• (countable) A decision-making method, especially one appropriate for a specialised realm.