formula
Formula (noun) means a rule in mathematics that is written using symbols rather than words. Example: “The teacher wrote the formula for the area of a circle on the board.”
Fill the gap
Can you complete this real example?
The teacher wrote the _____ for the area of a circle on the board.
Etymology
A learned borrowing from Latin fōrmula (“a small pattern or mold, form, rule, principle, method, formula”), a diminutive of forma (“a form”). See form.