the act of fulfilling the duties of any employment.

Function is a term used in analytics for an algebraical expression in any way compounded of a certain letter or quantity with other quantities or numbers; and the expression is said to be a function of that letter or quantity.

Thus $a - x$, or $ax + 3x^2$, or $2x - a\sqrt{a-x}$, or $x^2$, or $e^x$, is each of them a function of the quantity $x$.