Undefined may refer to:
Mathematics
- Undefined (mathematics), with several related meanings
Computing
- Undefined behavior, computer code whose behavior is not specified under certain conditions
- Undefined value, a condition where an expression does not have a correct value
- Undefined variable, a variable that is accessed in code but has not been declared
Other uses
- Undefined, something that lacks any definition
- Undefined citizenship, a post-Soviet form of statelessness in Estonia