re
preposition
uk/reɪ/ us/riː/ COMMUNICATIONS, INTERNET
used in the subject line of an email to show that you are replying to an email that was sent to you with the same title. If you reply to an email, this word is automatically added to the reply the person you are emailing receives:
The email was titled "Re: Our Meeting Next Week."
COMMUNICATIONS
used in business letters to refer to a letter or something in a letter you have received and are replying to:
Re your communication of 15 February...