Has an influence on or changes something (verb), or emotional feelings and moods (noun, psychology term).
From Latin 'afficere' (to influence or act upon); in psychology, 'affect' specifically refers to observable emotional expression, distinct from 'emotion' which is internal; the pronunciation differs between verb /əˈfɛkts/ and noun /ˈæfɛkts/.
Psychologists distinguish 'affect' (what others observe) from 'emotion' (what you feel), showing that language preserves important medical concepts—Shakespeare probably experienced 'flat affect' in depressed characters but lacked the terminology!
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