What does R stand for?

What does R mean?

R	Radiation Academic & Science » Physics
R	Radiation Academic & Science » Chemistry
R	Radical Governmental » US Government
R	Radical Governmental » Military
R	Radical Community » Educational
R	Radius Academic & Science » Mathematics
R	Radius Governmental » NASA
R	Rail Governmental » Military
R	Rail Governmental » NASA
R	Rail Governmental » Transportation
R	Rail Regional » Railroads
R	Railroad Governmental » Transportation
R	Railroad Regional » Railroads
R	Rain Academic & Science » Meteorology
R	Rain Academic & Science » Ocean Science
R	Raise Community » Sports
R	Rally Community » Sports
R	Ralphable Miscellaneous » Funnies
R	Ramming Governmental » Military
R	Rand (South Africa) Regional » Currencies
R	Random Academic & Science » Mathematics
R	Random Medical » Laboratory
R	Randy Miscellaneous » Names and Nicknames
R	Range Governmental » Military
R	Range Governmental » NASA