Of that or it; from or concerning that thing just mentioned. Used in formal writing to refer back to something previously stated without repeating it.
From Middle English thereof, combining 'there' (meaning 'of that') and 'of'. The construction follows the pattern of other formal adverbs like whereof and wherefore, common in legal and academic writing since the 14th century.
This word is like a linguistic shortcut that prevents awkward repetition in formal writing. Instead of saying 'the benefits of the policy and the costs of the policy,' you can elegantly write 'the policy and the benefits and costs thereof.'
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