The powerful ring made by Sauron in J.R.R. Tolkien’s The Lord of the Rings is called the One Ring.
Sauron forged the One Ring in the fires of Mount Doom during the Second Age. He placed much of his power into it so that he could dominate the other Rings of Power and control their bearers.
The Ring changes ownership several times in the narrative. Bilbo finds it beneath the Misty Mountains, gives it to Frodo, and Frodo carries it toward Mount Doom with Samwise Gamgee’s help. Its destruction ends Sauron’s power because the Ring is bound to him.
The One Ring is sometimes called simply “the Ring” in conversation, but that shorter term can be ambiguous outside the story. “The One Ring” is the specific canonical name used for Sauron’s master ring.