A sign, either real or imagined, of ill fortune or catastrophe in the future. It was a bad omen when my girlfriend broke up with me the day before my final exams. All of the recent natural disasters are a bad omen for the future of the planet.

A sign, either real or imagined, of good fortune or a propitious outcome. I saw a dove on my windowsill this morning; I'd say it's a good omen for the rest of my day. I think the way our conversation ended is a good omen for where our relationship is headed.