An omen is a sign that something will happen later — a bird, a dream, a strange little event that people read as news from the future. It can be a good omen or a bad omen, but when someone just says 'an omen', they often mean a worrying one. You hear it in stories, in talk about luck, and sometimes as a joke after a bad start ('that's a bad omen for the team'). There is a verb, to omen, but it is rare; you mostly see it in books. Most people only use the noun. Though actually, science does not treat omens as real knowledge. The word lives on in culture, films, and everyday talk about luck, not in the weather report.