Zarathushtra Spitama, more commonly known as Zoroaster or Zarathustra, was an Iranian religious reformer who challenged the tenets of the contemporary Ancient Iranian religion, becoming the spiritual founder of Zoroastrianism. In the oldest Zoroastrian scriptures, the Gathas, which he is traditionally believed to have authored, he is described as a preacher and a poet-prophet. He also had an impact on Heraclitus, Plato, Pythagoras, and the Abrahamic religions, including Judaism, Christianity, and Islam, particularly through concepts of cosmic dualism and personal morality.

He spoke an Eastern Iranian language, named Avestan by scholars after the corpus of Zoroastrian religious texts written in that language. Based on this, it is tentative to place his homeland somewhere in the eastern regions of Greater Iran (perhaps in modern-day Afghanistan or Tajikistan), but his exact birthplace is uncertain. His life is traditionally dated to sometime around the 7th and 6th centuries BC; though most scholars, using linguistic and socio-cultural evidence, suggest a dating to somewhere in the second millennium BC.

Zoroastrianism eventually became Iran's most prominent religion from around the 6th century BC, enjoying official sanction during the time of the Sassanid Empire, until the 7th century AD, when the religion itself began to decline following the Arab-Muslim conquest of Iran. Zoroaster is credited with authorship of the Gathas as well as the Yasna Haptanghaiti, a series of hymns composed in Old Avestan that cover the core of Zoroastrian thinking. Little is known about Zoroaster; most of his life is known only from these scant texts. By modern standards of historiography, no evidence can place him into a fixed period and the historicization surrounding him may be a part of a trend from before the 10th century AD that historicizes legends and myths.

His name likely means "he who manages camels," though its etymology has multiple interpretations, and the Greek form "Zoroaster" stems from later transliterations. According to Zoroastrian tradition, Zoroaster was trained as a priest from a young age and, around age 30, experienced a divine revelation introducing him to Ahura Mazda, the Wise Lord, and the dualism of truth (asha) versus deception (druj). He is said to have gained royal patronage under King Vishtaspa, spread his teachings, and founded a community, marrying three times and having six children. Zoroastrian texts portray his philosophy as emphasizing free will, ethical responsibility, and aligning with asha through good thoughts, words, and deeds.

Zoroaster's name in his native language, Avestan, was probably Zaraθuštra. His translated name, "Zoroaster", derives from a later (5th century BC) Greek transcription, Zōroastrēs (Ζωροάστρης), as used in Xanthus's Lydiaca (Fragment 32) and in Plato's First Alcibiades (122a1). This form appears subsequently in the Latin Zōroastrēs, and, in later Greek orthographies, as Ζωροάστρις, Zōroastris. The Greek form of the name appears to be based on a phonetic transliteration or semantic substitution of Avestan zaraθ- with the Greek ζωρός, zōros (literally 'undiluted') and the BMAC substrate -uštra with ἄστρον, astron, 'star'.

In Avestan, Zaraθuštra is generally accepted to derive from an Old Iranian *Zaratuštra-. The element half of the name (-uštra-) is thought to be the Indo-Iranian root for 'camel', with the entire name meaning 'he who can manage camels'. Reconstructions from later Iranian languages—particularly from the Middle Persian (300 BC) Zardusht,^{[further explanation needed]} which is the form that the name took in the 9th- to 12th-century Zoroastrian texts—suggest that *Zaratuštra- might be a zero-grade form of *Zarantuštra-. Subject then to whether Zaraθuštra derives from *Zarantuštra- or from *Zaratuštra-, several interpretations have been proposed.

If Zarantuštra is the original form, it may mean 'with old/aging camels', related to Avestic zarant- (cf. Pashto zōṛ and Ossetian zœrond, 'old'; Middle Persian zāl, 'old'):

The interpretation of the -θ- (/θ/) in the Avestan zaraθuštra was for a time itself subjected to heated debate because the -θ- is an irregular development: as a rule, *zarat- (a first element that ends in a dental consonant) should have Avestan zarat- or zarat̰- as a development from it. Why this is not so for zaraθuštra has not yet been determined. Notwithstanding the phonetic irregularity, that Avestan zaraθuštra with its -θ- was linguistically an actual form is shown by later attestations reflecting the same basis. All present-day Iranian-language variants of his name derive from the Middle Iranian variants of Zarθošt, which, in turn, all reflect Avestan's fricative -θ-.^{[citation needed]}