rarefied

Multiple Entries:
  rarefied    rarefy  

Concise Oxford English Dictionary © 2008 Oxford University Press:
rarefied /ˈrɛːrɪfʌɪd/

▶adjective

• 1 (of air) of lower pressure than usual; thin.

• 2 very esoteric or refined.

Concise Oxford English Dictionary © 2008 Oxford University Press:
rarefy /ˈrɛːrɪfʌɪ/ (also rarify)

▶verb (rarefies, rarefying, rarefied) make or become less dense or solid.

– derivatives
rarefication noun,
rarefactive adjective.

– origin ME: from OFr. rarefier, or med. L. rareficare, based on L. rarus ‘rare’ + facere ‘make’.