A supplement is something extra that is added to make up for a lack or to improve something. It can be extra pages in a book, vitamins in your diet, or additional material in a course.
From Latin 'supplementum' meaning 'that which fills up or completes', from 'supplere' meaning 'to fill up'. The idea has always been about filling a gap or adding what is missing.
Supplements are not just pills; extra reading at the end of a textbook is also a supplement. Whenever life, food, or information feels incomplete, humans tend to invent a 'supplement' to patch the hole.
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