The argument of a function is its input. For example, in the expression , is the argument of .

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 for , people who are unfamiliar with mathematics may give the erroneous answer . 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 regardless of what the function is.