An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been declared by that code.

In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time. In other languages such a usage is considered to be sufficiently serious that a diagnostic being issued and the compilation fails.

Some language definitions initially used the implicit declaration behavior and as they matured provided an option to disable it (e.g. Perl's "use warnings" or Visual Basic's "Option Explicit").