In re, Latin for "in the matter [of]", is a term with several different, but related meanings.
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Jurisprudence
In jurisprudence, in re: is used to indicate that a judicial proceeding may not have formally designated adverse parties or is otherwise uncontested. The term is commonly used in case citations of probate and bankruptcy proceedings, such as the General Motors Chapter 11 reorganization. It is sometimes used for consolidated cases, as with In re Marriage Cases. It was adopted by certain U.S. states like California when they adopted no-fault divorce to reflect the fact that the modern proceeding for dissolution of marriage was being taken out of the adversarial system. It is also used in juvenile courts, as, for instance, In re Gault. The Bluebook describes In re as a "procedural phrase", and requires using it to abbreviate "in the matter of", "petition of", "application of", and similar expressions.
Correspondence
In correspondence, the phrase in re: refers to the subject of a letter, memorandum, or electronic mail message (see RE (e-mail)). It is used especially in e-mail to denote in regard to; confusingly, RE: is employed to mean "in reply to:".
Philosophy
In philosophy, in re means "in reality", a statement about the real world as opposed to a statement about an ideal world – Plato's idea of philosopher kings, for example, has been criticized for being impossible to maintain in re.