Undefined may refer to:

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

• 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

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

• Null (disambiguation)
• Void (disambiguation)
• Invalid (disambiguation)
• Definition (disambiguation)
• Definable (disambiguation)
• All pages with titles beginning with Undefined