in Superman fiction, a glowing green mineral that weakens him; used generally to mean someone's critical weakness or vulnerability.
A science fiction neologism created for Superman comics in 1940, combining 'crypto' (hidden) and '-nite' (mineral suffix). It became so culturally powerful that the word entered everyday English to describe any fatal weakness.
Kryptonite is one of the rare examples of a word invented for fiction that became a legitimate English word used outside of Superman—it's an example of how pop culture can reshape language, especially through metaphor.
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