n 1  :  the second letter of the Greek alphabet - symbol B or b  2  :  BETA PARTICLE  3  :  BETA WAVE
or  b-  adj 1  :  of or relating to one of two or more closely related chemical substances <the beta chain of hemoglobin>  <b-yohimbine>  - used somewhat arbitrarily to specify ordinal relationship or a particular physical form and esp. one that is allotropic, isomeric, stereoisomeric, or sometimes polymeric (as in b- D-glucose)  2  :  second in position in the structure of an organic molecule from a particular group or atom : also  :  occurring at or having a structure characterized by such a position < substitution>   3  :  producing a zone of decolorization when grown on blood media - used of some hemolytic streptococci or of the hemolysis they cause .
 
 chiefly Brit   
 
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