Deviating from what is standard, normal, or expected; abnormal or irregular.
From Greek 'anomalos' (uneven, irregular, from 'an-' without + 'homalos' even). An older or more formal variant of 'anomalous,' now rarely used.
This is basically a fancier, older version of 'anomalous'—you'd see it in Victorian-era scientific writing but rarely in modern English, which usually picks the '-ous' ending instead.
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