Main Entry: rar·e·fac·tion

Pronunciation: \ˌrar-ə-ˈfak-shən, ˌrer-\

Function: noun

1 : the action or process of rarefying
2 : the quality or state of being rarefied ; especially : an increase in porosity <areas of rarefaction 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 rarefactions of the sound wave>

—rar·e·fac·tion·al \-shnəl, -shən-^əl\ adjective