An omen or sign is an event superstitiously believed to foretell the future.
In the mirror universe, after killing Admiral Black in 2155, Jonathan Archer announced that he intended to go "where the omens of the gods and the crimes of our enemies summon [them]": proceed to Earth and seize control of the Terran Empire. (ENT: "In a Mirror, Darkly, Part II")
In 2257, Kol-Sha proclaimed to the Klingon High Council that the red bursts were a bad omen for L'Rell's rule as Chancellor. (DIS: "Point of Light")
In 2370, Worf claimed to a group of Boraalans being unknowingly transported aboard the USS Enterprise that holodeck malfunctions were in fact a good omen called the "Sign of La Forge". (TNG: "Homeward")
In 2374, Captain Kathryn Janeway took a group of Borg cubes' leaving USS Voyager alone to be a good omen for their journey through Borg space. (VOY: "Scorpion") A year later, she called finding the USS Equinox's dedication plaque among the wreck of its bridge a good omen. (VOY: "Equinox")
In a story he told Icheb, Mezoti, Azan, and Rebi in 2377, Neelix noted that the crew should have interpreted turbulence inside a dark nebula to be an omen of the strange events to come. (VOY: "The Haunting of Deck Twelve")