The quality or state of being extreme; the degree to which something is furthest from the center or most intense.
From extreme + -ness (noun suffix creating abstract nouns). Standard English nominalization pattern turning the adjective 'extreme' into a noun describing the state or quality of being extreme.
The 'extremeness' of something is subjective—what's extreme to one culture (eating insects) is normal to another, which shows how this seemingly objective word is really about perspective!
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