Undefined may refer to:
Mathematics
- Undefined (mathematics), with several related meanings
- Indeterminate form, in calculus
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
- Undefined, a function or variable lacking a declaration
- Undefined, a variable lacking initialization
- Undefined, an unavailable linker symbol (function, or global variable)
Other uses
- Undefined, something that lacks definition
- Undefined citizenship, a post-Soviet form of statelessness in Estonia