R1 / är/ (also r) • n. (pl. Rsor R's) the eighteenth letter of the alphabet. ∎ denoting the next after Q in a set of items, categories, etc.PHRASES: the R months the months with R in their names (September to April), considered to be the season for eating oysters.the three Rs reading, writing, and arithmetic, regarded as the fundamentals of learning.R2 • abbr. ∎ rand: a farm worth nearly R1,3-million. ∎ Réaumur: 198.6 °R. ∎ Regina or Rex: Elizabeth R. ∎ (also ®) registered as a trademark. ∎ (in the U.S.) Republican: congressman Henry Hyde (R-Illinois). ∎ restricted, a rating in the Voluntary Movie Rating System that children under 17 require an accompanying parent or adult guardian for admission. ∎ (on a gearshift) reverse. ∎ (R.) River (chiefly on maps): R. Cherwell. ∎ roentgen(s). ∎ rook (in recording moves in chess): 21.Rh4.• symb. ∎ Chem. an unspecified alkyl or other organic radical or group. ∎ electrical resistance. ∎ Chem. the gas constant.