Rarefy v. t. : To make rare, thin, porous, or less dense; to expand or enlarge without adding any new portion of matter to; -- opposed to condense.

Rarefy v. i. : To become less dense; to become thin and porous.

Example:
what is the sound of one hand clapping?

Taxonomy
rarefy, rar, Merriam-Webster Online Dictionary, fy, Dictionary, webster, Word Games, Webster Online Dictionary with audio pronunciations, Middle English, glossary, language, ESL, rer, Inflected, fied, Etymology, Latin, rarus.

Defination from the web:
rarefy from the Merriam-Webster Online Dictionary with audio pronunciations, thesaurus, Word of the Day, and word games. ... Home. Visit Our Sites. Unabridged Dictionary. Learner's Dictionary. Word Central for Kids. Premium Services. Unabridged Dictionary. Encyclopedia Britannica ...