The constellation of Orion.
Orion's Belt is a string of stars in the Constellation of Orion, visible from Earth, which from left to right contain the stars Alnitak (Zeta Orionis), Alnilam (Epsilon Orionis), and Mintaka (Delta Orionis).
James T. Kirk once described this particular feature of the Earth night sky to Edith Keeler while he was trapped in the year 1930. (TOS: The City on the Edge of Forever)
Background
Captain Kirk described the belt, which allows us to know that the writer he spoke of originated in the Alnitak system, even though he didn't specify the name of the star he meant. The other stars in Orion's Belt are also in Star Trek: Mintaka was visited in TNG: "The Vengeance Factor." While Alnilam has no canon appearances, it is mentioned in star charts in Franz Joseph's Star Fleet Technical Manual by Ballantine Books (and even had a Constitution-class starship named after it, in his shiplists).