- 1曳光彈（醫學上的顯影劑／示蹤劑也叫 tracer）
A tracer is a bullet that burns as it flies, so it leaves a line of light or smoke in the air behind it. Soldiers load a few of them in among ordinary bullets — every fifth one, say — so that at night, or when they are shooting far away, they can see where their shots are going and correct their aim. The word is also used for the burning line itself: people talk about tracers going up into the sky. You will meet the word mostly in war films, war reports, and video games, and it is often used as 'tracer fire' or 'tracer rounds' rather than alone. Outside guns, the same idea gives 'tracer' another life in hospitals: doctors put a small amount of a substance into your body that a machine can follow, and that is also called a tracer. The thread running through all of it is the plain verb 'trace' — to follow a line, or to leave one that can be followed.
From the word’s history
From trace + -er.
trace（追蹤）加 -er：用來追蹤的東西或人。
組成trace（追蹤、留下痕跡）+ -er（做這件事的人或東西）。所以 tracer 就是「會留下痕跡的那個東西」。
原始詞典資料7 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，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。
tracernoun/ˈtɹeɪsə(ɹ)/
- 1
chemistrynatural-sciencesphysical-sciencesA compound, element, or isotope used to track the progress or history of a natural process.
- 2
A round of ammunition for a firearm that contains magnesium or another flammable substance arranged such that it will burn and produce a visible trail when fired in the dark.
- 3
The act or state of tracking or investigating something.
- We have a five-man tracer on him now. He's heading for a Vorster cell on Michigan Boulevard, and he's drunk as a lord. Should we intercept him?
- A surveyor typically conducts a tracer on his or her own and later meets up with the rest of the team to discuss findings.
- 4
A request to trace the movements of a person or an object, such as a shipment.
- The next morning when she awoke, she told Darcy what had happened, and Darcy agreed that she should go to the police and this time ask them if they could put a tracer on her incoming phone calls, just in case he called again.
- 5
A person who traces something.
- 6
A device or instrument used to assist in making tracings.
- 7
computingengineeringmathematicsA process that traces something, such as the path of execution of a program during debugging.
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。