What title did J.R.R. Tolkien give the creator deity of his fictional world?

J.R.R. Tolkien called the creator deity of his fictional world Eru Ilúvatar.

Eru Ilúvatar is the supreme being in Tolkien’s legendarium. In the creation account presented in The Silmarillion, he brings the Ainur into existence and gives them a great theme of music. The Ainur develop that theme through the Music of the Ainur, after which Eru gives independent being to the world, called Arda.

The name Eru means “The One,” while Ilúvatar is commonly translated as “Father of All.” Eru is not one of the Valar or Maiar; those beings are created spirits beneath him. Manwë, Melkor, and Aulë are therefore not alternative names for the creator, but powerful members of the created order.

Eru’s direct interventions are rare in the narrative. One notable example occurs when he changes the nature of the world after Númenor’s downfall. The concept gives Tolkien’s mythology a theological framework without making Middle-earth a direct retelling of any single real-world religious story.