in Analysis, denotes 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

\[ a - 4x, \text{ or } ax + 3x^2, \text{ or } 2x - a\sqrt{x^2 - x^2}, \text{ or } x^c, \text{ or } e^x, \]

is each of them a function of the quantity \( x \).