A portent is a sign that something big is going to happen, and it often feels dark. People reach for this word in books, history writing, and serious news more than at the dinner table. A strange sky before a storm can feel like a portent; a sudden quiet in a crowded street can too. It sits near omen, but a portent usually points at something large — a war, a crash, a turn in the world — not a small bit of luck. Though actually, writers sometimes use it more loosely, just to mean “this shows what will come next,” without the heavy feeling. It sounds a little literary, and a little old.

預兆、凶兆（偏書面）