Who was the Shah of Iran?

The origins of the term "Shah" go back to ancient Avestan "Kshathra" or Kshatrya" (Sanskrit) which means ruler of domain. The root of this word can also be found in the Iranian month of Shahrivar, itself a modern version of Kshathra Vairya, the Zoroastrian arch-angel of ideal dominion and guardian of metals and power among men.

Various dynasties used the term "Shah" to describe the rulers of their realm, including the Achamenid Persians, Arsacid Parthians and Sassanian Persians. The bactrian Kingdom ruling eastern Iranian land sin the time of Zarathushtra (Zoroaster) used the term "Kavi" for "King". So Shah is not an exact translation of "King" but more so a ruler of a domain or dominion.

In medieval Islamic times, Zoroastrian princes and rulers who continued to defy the Islamic Caliphate were known as "Sepahbodan" or leaders of armies, while Muslim rulers used the title of "Emir" and not Shah, until the Daylamite Buyids.

So it continued until the period from 1905 through 1911 when a constitutional monarchy was put in place, but it was overthrown in 1925. At that time, Reza Shah overthrew the dynasty to establish his own, and from that dynasty came the man we generally connect with the title. It is His Imperial Majesty, Mohammad Reza Pahlavi, Shah of Iran, that we usually refer to when we speak of the ruling monarch of Iran.

The Iranian Revolution of 11 February, 1979 overthrew the Shah of Iran and the Pahlavi dynasty.
Shah is a word for a king, from the Persian language. Over the years in the past, there were many shahs.

The last Shah was Mohammad Rezā Shāh Pahlavi, who was overthrown in 1979.