r

Multiple Entries:
  r    R    roentgen  

Concise Oxford English Dictionary © 2008 Oxford University Press:
r/ɑː/

▶abbreviation

• 1 recto.

• 2 right.

• 3 Law rule.

▶symbol

• 1 radius.

• 2 Statistics correlation coefficient.

Concise Oxford English Dictionary © 2008 Oxford University Press:
R1 (also r)

▶noun (pl. Rs or R's)

• 1 the eighteenth letter of the alphabet.

• 2 denoting the next after Q in a set of items, categories, etc.

– phrases
the R months the months September to April, considered to be the season for eating oysters.
the three Rs reading, writing, and arithmetic, regarded as the fundamentals of learning.

Concise Oxford English Dictionary © 2008 Oxford University Press:
R2

▶abbreviation

• 1 rand.

• 2 (in names of sports clubs) Rangers or Rovers.

• 3 Réaumur (a temperature scale, now obsolete).

• 4 Regina or Rex.

• 5 (®) registered as a trademark.

• 6 (in the US) Republican.

• 7 N. Amer. (in film classification) restricted (to viewers over a certain age).

• 8 (on a gear shift) reverse.

• 9 (R.) River.

• 10 roentgen(s).

• 11 Romania (international vehicle registration).

• 12 rook (in chess).

• 13 Cricket (on scorecards) run(s).

▶symbol

• 1 Chemistry an unspecified organic radical.

• 2 electrical resistance.

Concise Oxford English Dictionary © 2008 Oxford University Press:
roentgen /ˈrʌntjən, ˈrəːnt-, ˈrɒnt-/ (abbrev.: R)

▶noun a unit of ionizing radiation, the amount producing one electrostatic unit of positive or negative ionic charge in one cubic centimetre of air under standard conditions.

– origin 1920s: named after the German physicist and discoverer of X-rays Wilhelm Conrad Röntgen.