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Agreed, but negative luck is still luck in its own way.
You are more subjective on this matter. Same outcome for two people, same chance but still; one will be luckier than other because of our perceptions and desires.

I'd say that when a low probability undesirable event happens to you, then you are unlucky. I agree with the subjectiveness of luck. In fact, there have been cases where I thought I was unlucky due to an outcome, then later realized that I was actually lucky. This was because the outcome (in this case, not getting a specific job offer) seemed negative at the time, but later turned out to be for the best.

I know that feeling, happened so many times...
... and somewhere deep inside you knew it is going to turn out great...didn't you? At least thats how it goes for me usually.