prefix
Productive negative prefix attached to Sino-Japanese nouns and adjectives. Often corresponds to English un-, non-, in-, or dis-.
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This document has many deficiencies.
無 is also a negative prefix, but it often means 'without' or 'lacking' and attaches to a different set of words (e.g., 無関心 'indifference'). 不 tends to negate a quality or action directly.
非 means 'non-' or 'anti-' and often implies a stronger opposition or classification (e.g., 非公式 'unofficial'). 不 is a more straightforward negation.
From Middle Chinese 不 (pjuw). Used as a negative prefix in Sino-Japanese vocabulary.