Lacking a definition or value.
"The program crashed because it tried to divide by an undefined variable that had no assigned value."
In plain English: Undefined means something that has no clear value, meaning, or limit because it hasn't been defined yet.
"The programmer had to fix several undefined variables in his code before launching the app."
The word undefined is formed by adding the prefix un- to defined to mean not having a clear or specific meaning. It entered English as a straightforward combination of these two existing words rather than through borrowing from another language.