noun
undefined behavior; unspecified behavior
Computing term for program behavior not defined by the language specification; may produce unpredictable results.
C言語では、配列の範囲外アクセスは未定義の動作を引き起こす。
In C, accessing an array out of bounds causes undefined behavior.
未定義の動作に依存したコードは移植性が低い。
Code that relies on undefined behavior is not portable.
Compound of 未定義 (undefined) + の (possessive particle) + 動作 (behavior, operation). A direct translation of the English computing term 'undefined behavior'.