a. Zarathustra - 9. the founder of Zoroastrianism
Zarathustra, or more commonly referred to as Zoroaster, was an Iranian spiritual leader. It is not exactly when he lived and was spreading his teachings, but the estimates are that it was somewhere around the second millennium BC. This man was a spiritual leader as mentioned, and the foundations of the Zoroastrianism are attributed to him. He challenged the ancient Indo-Iranian religions with his religion and came out largely victories in it, as the Zoroastrianism very quickly was accepted and became the dominant religion among the Iranian people.
b. Amesha Spentas - 3. the ''Immortal Holy Ones'', or the Sacred Seven
Amesha Spentas is a term that has been used in the Zoroastrianism to describe the Immortal Holy Ones. They were divine entities in the Zoroastrianism and believed to be emanating from Ahura Mazda which was the highest divinity of this religion. Over time, multiple variations of the term have been used, depending on the language and the evolving of the words. The importance of Amesha Spentas was that they were the first seven emanations of the still uncreated creator, to whom the whole creation of the world was attributed.
c. Avesta - 4. the sacred text of Zoroastrianism
Avesta is a term used for the sacred texts of the Zoroastrianism. It is a collection of text written in Avestan language, a language that has no other proof of existence, and these texts are its only witness. The Avesta has been written is accordance to the purpose or usage of the texts, but there's also variations in the dialects, and it is easily noticeable that the writings are from several different dialects from several different periods. These texts were the holy book of the followers of the Zoroastrianism, just as the Bible is to the Christians, and the Koran to the Muslims, and everything about this religion can be found in them, from the creator, all of the divinities, the principles of it, the afterlife etc.
d. Spenta Mainyu - 2. the Zoroastrian name for a holy spirit
Spenta Mainyu is a term used in the Zoroastrianism to describe a holy spirit. This Holy Spirit was created by the wise God Ahura Mazda. The reason why Ahura Mazda created Spenta Mainyu was to counter and oppose the evil spirit Angra Mainyu. Spenta Mainyu is considered to actually be part of Ahura Mazda himself, and through it he was creating life and doing goodness. The Holy Spirit that Spenta Mainyu was is attributed to maintaining and protecting numerous reals, including the earth, waters, the sky, the children that are yet to be born, plants etc.
e. Ahura Mazda - 7. ''God the Eternal Light''
Ahura Mazda is considered as the creator in the Zoroastrianism. He is the highest deity in this religion and it is no wonder that it is the character that is by far the most mentioned in the sacred texts. Ahura Mazda though hasn't been included in the religion until the Achaemenid Period. During this period he was alleviated to the title of creator and was worshiped by the followers of the Zoroastrianism. Later in history, Ahura Mazda was included as a part of a triad together with Anahita and Mithra. Interestingly enough there is not a representation of Ahura Mazda, and it is only an empty chariot drawn by white horses in the imperial court, with the purpose of calling this deity to join the Persian Army when going in war.
f. Maidhynimaonha - 8. Zoroaster's first convert
Maidhynimaonha, according to the Avesta texts, is considered to be the first person that Zarathustra converted to his newly created religion the Zoroastrianism. This person, similar to Zarathustra and Vishtaspa, is not a person that is confirmed to have existed, thus to be a real historical person, so he is also something in between myth and reality. He was the first person that found the Zoroastrianism appealing, so he gladly converted to it. Once converted, he become one of the closest friends of Zarathustra, and one of his disciples, and together they were spreading around the Zoroastrianism, creating the initial core of followers of it.