Merriam-Webster Collegiate Dictionary - mitigate

 transitive verb  (-gated; -gating)  Etymology: Middle English, from Latin mitigatus, past participle of mitigare to soften, from mitis soft + -igare (akin to Latin agere to drive); akin to Old Irish moith soft — more at agent  Date: 15th century  1. to cause to become less harsh or hostile ; mollify aggressiveness may be ~d or…channeled — Ashley Montagu  2.  a. to make less severe or painful ; alleviate  b. extenuate  Synonyms: see relieve  • mitigation noun  • mitigative adjective  • mitigator noun  • mitigatory adjective Usage:  Mitigate is sometimes used as an intransitive (followed by against) where militate might be expected. Even though Faulkner used it some intangible and invisible social force that ~s against him — William Faulkner and one critic thinks it should be called an American idiom, it is usually considered a mistake.