One way to picture it: something inside a body is let out and travels away — smoke leaving a chimney, light leaving a lamp, heat leaving an engine. The word covers both the letting-out itself and, in the plural, the stuff that comes out.
From the word’s history
First attested 1607, from Middle French émission, from Latin ēmissiō 'a sending forth', from ēmittō — ex- ('out of') + mittō ('send'). The same mittō gives English mission, transmit and submit.
1607 年首見於英文，來自中古法文 émission，再往上是拉丁文 ēmissiō（送出去），由 ēmittō 而來——ex-（向外）加 mittō（送）。同一個 mittō 也給了英文 mission、transmit、submit 這些字，全部都藏著「送」的動作。知道這一點之後，「排放」就不再是一個要死背的環保單字：工廠排放廢氣，字面上不過就是「把東西往外送」而已。
Entry origin note: Middle French émission < Latin ēmissiō < ēmittō (ex- + mittō)
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原始詞典資料3 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。
emissionnoun詞源 1/ɪˈmɪʃ.ən/
- 1
Something which is emitted or sent out; issue.
- the emission was mostly blood
- Cymarron's sound resembled the mellow folkie emissions of Crosby, Stills, Nash & Young and America.
- 2
The act of emitting; the act of sending forth or putting into circulation.
- the emission of light from the sun
- the emission of heat from a fire
emissionnoun詞源 2
- 1
broadcastingmediaA show; a program.
- All too often, such shows result in destroying any idea that the topic just _could possibly_ be serious. I also discussed with gothic friends, telling they once were interviewed by people claiming to be creating an emission about gothics... just to discover later that the real topic was sects!
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。