n 1  :  the action or process of rarefying  2  :  the quality or state of being rarefied : esp  :  an increase in porosity <areas of in the bones>   3  :  a state or region of minimum pressure in a medium transversed by compressional waves (as sound waves) <the eardrum vibrates according to the compressions and s of the sound wave>  rar*e*fac*tion*al adj 
 
   
 
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