- 1
In science, the half-life of a radioactive substance is how long it takes for half of it to decay — to lose half of its . After one half-life, half of the original amount is still there. After another half-life, half of that remaining half is left, so a of what you started with. It does not jump to zero; it keeps halving, again and again.
In science在科學裡，of a radioactive substance一種放射性物質，the half-life的半衰期，is how long it takes就是需要多久，for half of it to decay它的一半才衰變掉——to lose half of its radioactivity也就是失去一半的放射性。After one half-life過了一個半衰期，half of the original amount原本份量的一半，is still there還在那裡。After another half-life再過一個半衰期，half of that remaining half剩下那一半的一半，is left留了下來，so a quarter of what you started with也就是最初的四分之一。It does not jump to zero它不會一下子跳到零；it keeps halving它不斷對半減下去，again and again一次又一次。
Different materials have wildly different half-lives: some last seconds, some last thousands of years. Scientists like this number because it is a steady clock.
Different materials不同的材料，have wildly different half-lives半衰期差得非常遠：some last seconds有的只撐幾秒，some last thousands of years有的撐幾千年。Scientists科學家 like this number喜歡這個數字，because因為 it is a steady clock它是一座走得很穩的時鐘。
Though actually, the “life” in the word is not about being alive. It is about how long the substance lasts as itself.
Though actually不過實際上，the “life” in the word字裡的 “life”，is not about being alive和活著沒有關係。It is about它講的是 the substance這個物質 as itself以自己原本的樣子 lasts how long能撐多久。
People also stretch the word in ordinary talk: the half-life of a song, a , or a news story is how long until it has lost half of its heat and most people have moved on. That second use is informal. In class and in the lab, half-life means the science sense.
in ordinary talk日常談話裡，People also stretch the word人們也把這個字拉開來用：of a song, a rumour, or a news story一首歌、一則謠言或一條新聞，the half-life的半衰期，is how long until就是要多久，it has lost half of its heat它掉了一半的熱度、and most people have moved on多數人已經往別的事去了。That second use第二種用法 is informal是非正式的。In class and in the lab在課堂和實驗室裡，half-life means指的就是 the science sense科學上的那個意思。
組成From half- + life. Here “life” means how long something lasts, not a living person.
同家族的字
- lifethe second piece; in this word it means a span of time, not a living body
原始詞典資料4 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。
half-lifenoun/ˈhæfˌlaɪf/
- 1
The time required for half the nuclei in a sample of an isotope to undergo radioactive decay.
- "The guy that just left—what's he got in his lungs?" / "Well, I'm not very sure. But the two best candidates are uranium-233 and plutonium-239, one or both." / "They're in his bones too, aren't they?" / "Yeah, they're both boneseekers. And they've got half-lives of one hundred sixty-two thousand and twenty-four thousand years respectively. So they stay hot for a long time."
- O₂^(•−) and OH^• are highly unstable (half-life at the level of microseconds and nanoseconds, respectively) and cannot cross membrane, while H₂O₂, though not a free radical but a ROS, is relatively stable (half-life around 1 ms) (Møller et al. 2007) and can cross membranes through aquaporins.
- 2
chemistrynatural-sciencesphysical-sciencesIn a chemical reaction, the time required for the concentration of a reactant to fall from a chosen value to half that value.
- 3
medicinepharmacologysciencesThe time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacological, physiologic, or radiological activity.
- For nonporous surfaces such as steel, in a dark and low-humidity environment, the CCP virus has an 18-hour half-life—the time required for it to decrease by half , according to the researchers' findings.
- 4
The amount of time it takes for an idea or a fashion to lose half of its influential power.
- Most books of scholarship have surprisingly short intellectual half-lives during which they make a difference"
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。