A transitional phrase meaning 'also' or 'as well', used to introduce supplementary information that adds to what has already been stated.
From Latin 'additionem' meaning 'something added to'. The phrase became common in English during the Renaissance as Latin-influenced formal writing styles developed, offering a more elaborate alternative to simple words like 'also'.
This phrase demonstrates English's love affair with Latin formality - we could simply say 'also' but often choose the more elaborate 'in addition' for emphasis or formality. The mathematical metaphor (addition) shows how we conceptualize information accumulation as arithmetic operations.
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