[in] the matter of
More literally, "by the thing". From the ablative of res ("thing" or "circumstance"). It is a common misconception that the "Re:" in correspondence is an abbreviation for regarding or reply; this is not the case for traditional letters. However, when used in an e-mail subject, there is evidence that it functions as an abbreviation of regarding rather than the Latin word for thing. The use of Latin re, in the sense of "about, concerning", is English usage.