The Collaborative International Dictionary of English v.0.48:

Zarathustra \Zar`a*thus"tra\, Zarathuster \Zar`a*thus"ter\,
   prop. n.
   The Persian name for Zoroaster.
   [PJC] Zarathustrian

The Collaborative International Dictionary of English v.0.48:

Zoroaster \Zo`ro*as"ter\, prop. n.
   The persian prophet Zarathustra, who probably lived in the
   second millenium B. C. His treachings form the basis for the
   religion Zoroastrianism.
   [PJC]

WordNet (r) 3.0 (2006):

Zarathustra
    n 1: Persian prophet who founded Zoroastrianism (circa 628-551
         BC) [syn: Zoroaster, Zarathustra]