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"Undefined" means something has not been clearly explained, given a fixed meaning, or given a fixed value. In everyday talk, you might say the rules of a game are "undefined" if nobody explained them clearly, or someone's job role is "undefined" if it's unclear what they're actually supposed to do. In math and computer programming, "undefined" has a very exact, technical meaning: it describes something that has no valid value at all, like dividing a number by zero - the answer is simply not defined.
未定義的、不明確的
組成"Un-" (not) + "defined" (given a clear meaning or limit) - so, "not given a clear meaning."
原始詞典資料3 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
undefinedadj
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Lacking a definition or value.
- We slid into our office, past two women discussing the "X factor," an apparently undefined quality their firm felt they lacked.
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computingengineeringmathematicsThat does not have a meaning and is thus not assigned an interpretation.
- The result of division by zero is undefined.
undefinednoun
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computingengineeringmathematicsAn undefined value.
- [Option:] · [PDP11:] F [BSD:] · [XE 2.3:] [1] [XE 3.0:] · [SIII:] · [COFF:] · [meaning:] make undefineds global
- Every operation is defined completely either by delivering a value of some type depending only on the types of its operands or by causing some defined exception; there are no ‘undefineds’ in Ten15.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。