n. A person who belittles the worth of another person or cause.
the GNU version of the Collaborative International Dictionary of English
n. One who detracts; a derogator; a defamer.
The Century Dictionary and Cyclopedia
n. One who detracts, or takes away or injures the good name of another; one who attempts to disparage or belittle the worth or honor of another. Sometimes written detracter.