The argument of a function is its input. For example, in the expression $f(x)$ , $x$ is the argument of $f$ .

A common error for those who are unfamiliar with mathematics is to treat a function and its argument as two separate entities. For example, in solving the equation $\ln x=5$ for $x$ , people who are unfamiliar with mathematics may give the erroneous answer $\displaystyle x=\frac{5}{\ln}$ . This error might be circumvented by stressing that a function and its argument are not multiplied, but rather that a function acts on its argument.

Another common error is to try to separate the argument of a function. This error is most common when the argument consists of at least two terms. For example, students may write $f(x+5)=f(x)+f(5)$ regardless of what the function $f$ is.