极端 jíduān extreme

How to remember 极端: 极 extremely + 端 end = extreme.

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Extreme - the far end of a spectrum: 极端天气 (jíduān tiānqì) extreme weather, 极端分子 (jíduān fènzǐ) extremists, 走极端 (zǒu jíduān) go to extremes.
Both adjective (extreme/radical) and noun (an extreme); often carries a negative sense of excess.
Contrast 极为 (jíwéi, extremely, an intensifier); 极端 is the extreme itself or being radical.