from The American Heritage® Dictionary of the English Language, 4th Edition
- transitive v. To make thin, less compact, or less dense.
- transitive v. To purify or refine.
- intransitive v. To become thin or less compact or dense.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- v. To make rare, thin, porous, or less dense
- v. To expand or enlarge without adding any new portion of matter to.
from the GNU version of the Collaborative International Dictionary of English
- transitive v. To make rare, thin, porous, or less dense; to expand or enlarge without adding any new portion of matter to; -- opposed to condense.
- intransitive v. To become less dense; to become thin and porous.
from The Century Dictionary and Cyclopedia
- To make rare, thin, porous, or less dense; expand or enlarge without adding any new matter; figuratively, to spread or stretch out; distend: opposed to condense.
- To become rare; pass into a thinner or less dense condition.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- v. lessen the density or solidity of
- v. weaken the consistency of (a chemical substance)
- v. make more subtle or refined
Middle English rarefien, from Old French rarefier, from Medieval Latin rārificāre, alteration of Latin rārēfacere : rārus, rare + facere, to make; see dhē- in Indo-European roots.(American Heritage® Dictionary of the English Language, Fourth Edition)
rare + -fy (Wiktionary)