Re: is one of those commonly used letter combinations (like SIC) that people tend to make up their own meanings for.
Re: is used at the top of letters and emails in order to steer the reader to the single most important topic of the message:
Re: Your order of 10/3/09
Re: Your submission For Whom the Bell Tolls
I’ve seen Re: explained as an abbreviation of the words “regarding” or “referencing.”
However, Re is not an abbreviation for anything.Re: means “re.”
Re is an English preposition in use since at least the 18th century. It means “in the matter of, with reference to.”
Like sic, re is a Latin word. It is the ablative form of the Latin noun res meaning “thing” or “affair.” Lawyers use the legal phrase in re when a proceeding is not brought by a person, but has to do with something like probate, or a public project like laying out a highway.
NOTE: Watch out for the definition “in regards to” given at Wiki Answers.
“In regards” is nonstandard English for in regard to.