Something that is different from what is normal or expected, especially in a way that seems unusual or wrong.
From Latin 'ab-' (away from) and 'norma' (rule), plus '-ity' (state of). The modern sense developed in the 1800s with the rise of statistical thinking about 'normal' populations.
The concept of 'abnormality' is surprisingly modern—before statistics, people just said things were 'rare' or 'unusual'—and defining what's 'normal' often reflects prejudice more than actual science.
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