rarefied

adj. esoteric; elite; meant only for the initiated or the select few; very refined; of or pertaining to a select group; exalted, elated, euphoric, in high spirits; of extremely high quality in character or style (Literature); having low density, of lower pressure than usual

v. thin out, make less dense; refine, purify

Rarefaction is the reduction of a medium's density, or the opposite of compression. A natural example of this is as a phase in a sound wave or phonon. Half of a sound wave is made up of the compression of the medium, and the other half is the decompression or rarefaction of the medium.Another natural example of rarefaction is in the layers of our atmosphere. Because what constitutes our atmosphere has mass, it is definite that most of the atmospheric matter will be nearer to the Earth. Therefore, air at higher layers of the atmosphere is considered to have less pressure, or is rarefied in relation to air at lower layers.
See more at Wikipedia.org...

Adjective
1. having low density; "rare gasses"; "lightheaded from the rarefied mountain air"
(synonym) rare, rarified
(similar) thin
2. reserved for an elite group
(synonym) rarified
(similar) reserved
3. of high moral or intellectual value; elevated in nature or style; "an exalted ideal"; "argue in terms of high-flown ideals"- Oliver Franks; "a noble and lofty concept"
(synonym) exalted, high-flown, high-minded, lofty, rarified, idealistic, noble-minded
(similar) noble

Verb
1. lessen the density or solidity of; "The bones are rarefied"
(hypernym) change, alter, modify
(derivation) rarefaction
2. make more subtle or refined
(synonym) sublimate, subtilize
(hypernym) change, alter, modify
3. weaken the consistency of (a chemical substance)
(synonym) attenuate
(hypernym) weaken
(derivation) rarefaction
(classification) chemistry, chemical science

(imp. & p. p.)
of Rarefy
  

Synonyms and related words:
adulterated, air-built, airy, attenuate, attenuated, boyish, chimerical, clannish, cliquish, cloud-built, cut, dainty, delicate, diaphanous, dilute, diluted, dreamlike, elevated, esoteric, ethereal, exalted, exclusive, fanciful, fatuitous, fatuous, fine, fine-drawn, finespun, flimsy, frail, gaseous, gauzy, girlish, gossamer, gossamery, gracile, high, illusory, imaginary, insubstantial, lacy, lean, light, lofty, misty, noble, papery, phantomlike, private, rare, reduced, scant, scanty, select, shadowy, slender, slenderish, slight, slight-made, slim, slimmish, slinky, small, sparse, spirituous, sublime, subtile, subtle, svelte, sylphlike, tenuous, thin, thin-bodied, thin-set, thin-spun, thinned, thinned-out, thinnish, threadlike, uncompact, uncompressed, unreal, unsubstantial, vague, vaporous, wasp-waisted, watered, watered-down, watery, weak, willowy, windy, wiredrawn, wispy