See also Undefined (mathematics)

Undefined may refer to:

• 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)

• Undefined (mathematics), with several related meanings
  • Indeterminate form, in calculus

• Undefined, something that lacks definition
• Undefined citizenship, a post-Soviet form of statelessness in Estonia

• Null (disambiguation)
• Void (disambiguation)
• Invalid (disambiguation)
• Definition (disambiguation)
• Definable (disambiguation)