1. a lucky sign or something which could be considered an indication of future good performance or outcome
a good sign/omen of things to come
A good omen could be something superstitious considered to be a good or lucky sign, or a physical indication of a positive outcome in the future. You can use the terms 'good sign' or 'good omen' interchangeably, but 'omen' has a more superstitious or symbolic feel to it. You might think of it as more poetic. Generally, you say 'a sign OF', but 'an omen FOR'.