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Found 2 definitions 

  1.                 From Japanese-English FreeDict Dictionary ver. 0.1:
                

    
                
                  
    Ｒ, ｒ, アール
1. (noun (common) (futsuumeishi))
R, r
2.  [mathematics]  (esp. ｒ)
radius
3.
         Note: word usually written using kana alonecurve

    
                
              
  2.                 From Japanese-English FreeDict Dictionary ver. 0.1:
                

    
                
                  
    Ｒ, ｒ, アール
1. (noun (common) (futsuumeishi))
R, r
2.  [mathematics]  (esp. ｒ)
radius
3.
         Note: word usually written using kana alonecurve