WordWeb Online

1. Lessen the density or solidity of
   "The bones are rarefied"
    
2. Make more subtle or refined
   - sublimate, subtilize, subtilise [Brit]
    
3. Weaken the consistency of (a chemical substance)
   - attenuate

Derived forms: rarefied, rarefies, rarefying

See also: rarefaction

Type of: alter, change, modify, weaken