Injection嘛，我们第一反应肯定是注射。注射，简单而言，就是将液体注入皮下组织、血管或者器官。
以往都是从普通解释，引申到医学领域的用法。今天，我们反其道而行之。
An act of injecting sb with a drug or substance.
Oxford 8th
If you have an injection, a doctor or nurse puts a medicine into your body using a device with a needle called a syringe
Collins
(1) : the act or an instance of injecting a drug or other substance into the body
(2) : a solution of a drug, nutrient, or other substance injected (as by catheter or needle) into the tissues, a vein, or a body cavity
(3) : a solution or suspension of a drug intended for administration under or through the skin or mucous membranes by means of a hypodermic syringe
(4) : an act or process of injecting vessels or tissues; also : a specimen prepared by injection — compare corrosion
(5) : the state of being injected : congestion
Webster
The action of or an act of driving or forcing (a fluid) into a passage, cavity, or solid material under pressure; esp. in Medicine, introduction of a medicine, preservative, etc., by means of a (hypodermic) syringe.
SOED5
- [countable] a drug or another substance that is injected into your body
1a. [uncountable] the process of injecting a drug or another substance into your body
Macmillan
上述大体上，是我常用的基本英英词典里的解释。大家阅读之后也会发现，所谓的injection，既可以是动作，也可以是液体，真是还可以表示过程。
（顺带小声说一句，看看英英词典；想想自己以前看英汉词典时，天下我有的豪迈是否消失了呢？）
插入个分割线，我们继续聊医学以外的injection。
先看牛津
- [C] a large sum of money that is spent to help improve a situation, business, etc. 大量资金的投入
The theatre faces closure unless it gets an urgent cash injection. 剧院面临着倒闭，除非有大笔救急现金投入。
- [U, C] an act of forcing liquid into sth （液体的）注入，喷入
a fuel injection system 燃油注入装置
从词典里，我们学到了至少两个知识点。
- 资金投入，cash injection
- help 和 improve之间有to，但通常省略掉了
再来瞧瞧麦克米兰
the addition of money to a business, a government programme, or someone's bank account
injection of: The company needs a huge injection of cash, and better trained staff.
the need for injection of capital from the parent bank
韦伯斯特
the placing of an artificial satellite or a spacecraft into an orbit or on a trajectory; also : the time or place at which injection occurs
随便翻翻词典，我们似乎就能发现许多小知识，翻译不就是这样么？一点又一点，慢慢知道的就多了。