R.
R. is an abbreviation of the Latin word Rex (King) or Regina (Queen).
Jurisprudence
In jurisprudence, it is used as notation in British or other Commonwealth realm criminal prosecutions to mean "the Crown" or "the state", which is represented by the current monarch.
It is often seen written as "R. v Defendant" which would be read as "The Crown against Defendant".
In jurisdictions that are republics, there is no Monarch and no one person embodies the state, so the prosecuting party may either be the Public Prosecutor or Director of Public Prosecutions, the State in and of itself, or (in some locations) the state on the behalf of the people it governs. An example of the first would be in Singapore, where cases are titled Public Prosecutor v Defendant; in Finland the expression is "syyttäjä vastaan X", "[public] prosecutor vs. X". An example of the second would be federal criminal cases in the United States (such as, for instance, the murder of persons in a building owned or operated by the United States government), which being prosecuted by the United States federal government would be termed United States v Defendant. On the other hand, most U.S. states, when prosecuting an offense, typically couch their case in terms of representing the people of the state, so if John Doe robbed a bank in Detroit, the case would be referred to as the People of the State of Michigan v John Doe.
Historiography
In historiography, r. can be used to designate the ruling period of a person in dynastic power, to distinguish from his or her lifespan.
For example, one may write "Charles V (r. 1519–1556)" instead of "Charles V (1500–1558)" if the writer considers the year of enthronement to be more important information for the reader than the year of birth, or occasionally to emphasise when a ruler abdicated before dying. In addition to rex or regina, r. can also be an abbreviation of regnavit ("[he/she] ruled").