The constellation of Orion.
Orion is a stellar constellation, visible from Earth, containing the stars Betelgeuse, Rigel, Bellatrix, Alnitak (Zeta Orionis), Alnilam (Epsilon Orionis), and Mintaka (Delta Orionis), as well as the Orion Nebula. Orion's Belt is the string of stars within the constellation, which from left to right, contain the stars Alnitak Alnilam and Mintaka.
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. Minktaka III, a planet orbiting Mintaka, was visited in "Who Watches the Watchers?" (TNG).
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).