Merriam-Webster Collegiate Dictionary - etymon

 noun  (plural etyma; also -mons)  Etymology: Latin, from Greek, literal meaning of a word according to its origin, from etymos true; akin to Greek eteos true — more at sooth  Date: circa 1576  1.  a. an earlier form of a word in the same language or an ancestral language  b. a word in a foreign language that is the source of a particular loanword  2. a word or morpheme from which words are formed by composition or derivation