Something which portends or is perceived to portend either a good or evil event or circumstance in the future, or which causes a foreboding; a portent or augury.
The ghost's appearance was an ill omen.
A rise in imports might be an omen of economic recovery.
3 more examples
The egg has, during the span of history, represented mystery, magic, medicine, food and omen.
Day broke. He saw three black hens asleep in a tree. He shuddered, horrified at this omen. Then he promised the Holy Virgin three chasubles for the church, and that he would go barefooted from the cemetery at Bertaux to the chapel of Vassonville.1856, Gustave Flaubert, chapter 10, in Eleanor Marx-Aveling, transl., Madame Bovary, Part 3:
Necessarily she took it for an omen.1924, Arnold Bennett, Riceyman Steps, page 222: