先擋住疾病
這一支是這個字的本行。要擋的東西是疾病，時間點永遠在生病之前。形容詞說「這是預防性的」，名詞就直接把那樣東西叫出來。
adjective · C1
Used to describe medicine, treatment or action that is given or taken to stop an illness before it starts, rather than to deal with one that has already begun.
預防性的
核心畫面
One picture covers every use: something already standing in the way before the trouble arrives. On an ordinary day it looks useless — the antibiotic taken the morning before the tooth comes out, the mask in the drawer, the rule nobody has had to apply, the backup that runs at eleven and reports nothing. You only find out what it was for on the day the trouble shows up and does not get through. Everything this word does is a version of that: it is never about fixing, always about being early.
這個字可以想成一樣東西，在麻煩來之前就已經站在路上擋著。平常看起來沒什麼用：拔牙前一天吃的抗生素、抽屜裡的口罩、一條…更多收起
這個字可以想成一樣東西，在麻煩來之前就已經站在路上擋著。平常看起來沒什麼用：拔牙前一天吃的抗生素、抽屜裡的口罩、一條還沒用上的規定、每晚十一點自己跑完的備份。你要等到麻煩真的來、而且沒進來的那一天，才知道它一直在做什麼。這個字的每一種用法都是同一件事：它從來不談修補，只談「早一步」。From the word’s history
Wiktionary gives prophylactic as from Latin prophylacticus, from Ancient Greek prophulaktikós. The packet supplies the route only and no meaning for the Greek parts, so the guard-standing-in-front picture above is offered as a teaching image, not as a claim about the root.
Wiktionary 記載 prophylactic 來自拉丁文 prophylacticus，再往前是古希臘文 προφυλακτικός。手邊的資料只給了這條來路，沒有給希臘字各部分的意思。所以上面那個「先站在前面擋著」的畫面是教學用的想像，不是對字根的主張。
Wiktionary entries 'prophylactic' (noun and adj), source_refs kaikki:3f6f69a190036ae4:657652982:12279 and :657665261:7150, supplied verbatim in the evidence packet
Three branches
先擋住其他壞結果
把要擋的東西從疾病換成別的麻煩，畫面完全沒動：一樣是事前架好、平常看不出作用的東西。
noun · C2
Something set up in advance to stop a bad result of any kind — a law, a rule, a system or a habit that stands in the way of trouble before it arrives.
預防措施／防範機制
美式英文裡的保險套
同一個「事前擋住」的畫面用在性健康上。它跟核心的距離其實很近，特別的是語域：這是美式英文選來寫在表格和標示上的字。
noun · C2
In American English, a formal word for a condom. It is often chosen to sound official or polite, and sometimes it is used as a joke because the word is so long and technical.
保險套