Definition of re preposition from the Oxford Advanced American Dictionary

 

re2

 preposition
preposition
NAmE//ri//
 
, NAmE//reɪ//
 
 
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used at the beginning of a business letter, etc. to introduce the subject that it is about; used on an e-mail that you are sending as a reply Re your letter of September 1… Re: travel expenses
See the Oxford Advanced Learner's Dictionary entry: re