One way to picture it: something is there, but the work it would do is already being done. A second key to the same lock, a sentence that repeats what the sentence before it said — nothing is wrong with the thing itself, there is simply no gap left for it to fill.
From the word’s history
From Latin *redundare* 'to overflow', built from *unda* 'wave' — the picture of water rising past the edge of a container. What overflows is not bad water; it is simply more than the vessel has room for.
來自拉丁文 redundare「滿溢、漫出來」，字根 unda 就是「波浪、水」。原始畫面是水裝到超過容器邊緣、往外漫出去。漫出來的水並不是髒水，只是容器已經裝不下、用不到了——這正好解釋現代的用法：redundant 說的不是東西壞掉，而是超出了實際需要的量。
Online Etymology Dictionary / Oxford English Dictionary (redundant < Latin redundare 'to overflow', from re- + unda 'wave')
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原始詞典資料5 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。
redundantadj/ɹɪˈdʌn.dənt/
- 1
Superfluous; exceeding what is necessary, no longer needed.
- It is allowed, that Senates and great Councils are often troubled with redundant, ebullient, and other peccant Humours, with many Diſeaſes of the Head and more of the Heart; […]
- In the living state, the body is observed to receive aliment; to assimilate a part; to evacuate what is redundant or useless; […]
- 2
communicationsjournalismliteratureRepetitive or needlessly wordy.
- 3
Dismissed from employment because no longer needed.
- Joyce Barnaby: You don't kill someone because they made you redundant. DCI Tom Barnaby: Not in the town, dear, but this is the country. And, in the country, anything goes.
- Four employees were made redundant.
- 4
Duplicating or able to duplicate the function of another component of a system, providing backup in the event the other component fails.
- The two lines are mainly used for redundant and therefore fault-tolerant message transmission, but they can also transmit different messages.
- 5
computingengineeringmathematicsContaining duplicate pathways to send a message.
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。