Main Entry: rar·e·fy

Variant(s): also rar·i·fy \ˈrer-ə-ˌfī\

Function: verb

Inflected Form(s): rar·e·fied; rar·e·fy·ing

Etymology: Middle English rarefien, rarifien, modification of Latin rarefacere, from rarus rare + facere to make — more at do

Date: 14th century

transitive verb 1 : to make rare, thin, porous, or less dense : to expand without the addition of matter
2 : to make more spiritual, refined, or abstruseintransitive verb : to become less dense