I use the term "luck" rhetorically to describe subjective perceptions only, which I do not believe have any basis in objective reality. I am convinced that there is no such thing as luck in the external world.

Good and bad are judgments you make according to how you react. Often, it depends on the agency of what affected us. If we unluckily get punched by a drunken guy, we call it bad, even if we meet the love of our life nursing us later. But then later, we might call the same event bad. The same reasoning can apply to any situation, because you can always switch points of view to make a situation good or bad no matter who’s looking at it.

There is a story about a man who had a horse and a son. Long story short. One day a thief stole the horse. People in the village were sorry for him, "What a bad luck." He answered, "Bad luck, good luck, who knows." His son tried to find the thief, didn't succeed. Coming back home through the forest he caught two wild horses. People in the village said "What a good luck!" The man said "Good luck, bad luck, who knows." While riding the wide horses the son fell off and broke his leg. People said, what a bad luck. The man, "Bad luck, good luck, who knows." It was war time. The son got summoned to the army, he couldn't go with his leg broken. "What a good luck!!"

Well, it is merely a social construct, and can mean whatever. Perhaps one can perceive "luck" as being when an event that had extremely little probability of occurring; occurs and it favors an individual. For whom ever it does not favor it would be "bad luck." Then again as I said to begin with it comes down to perceptions of events. One more crazy story as an example. Paraphrasing a real story I remember reading about in the Khaneman's book. Circa 1940 in Poland—I believe it was—a woman had taken a picture of her toddler and went to a store where you could get the negatives developed.

The second World War broke out rapidly and the mother had to flee the city, without getting the developed picture of her son. After many years in the new city she lived in, she now had received a second son, and had yet again taken photos of him she wanted developed. Here comes the twist—when she received the developed pictures of her second son, one could clearly see the first son in the picture, transparent! Somehow, the original picture of her first son never got developed 'cause of WWII, and was most likely misplaced into a box of "unused" pictures, and later sold only to "magically" end up in her new city and "accidentally" the photo paper reused. Sounds WAY to good to be true, yet it is a true story used as an example in the book. One could easily attribute it to "divine help," "destiny" or "meant to be luck" and most likely have the majority comply to your perspective. Yet the authors reminds us that no matter how small a probability a given event has of occurring, it still will occur from time to time, and it has nothing to do with "luck." Every event has it probability of occurring, small or big. And when these unlikely ones happen, people usually jump to high order explanations.

*) Kahneman and Tversky's book: "Judgement and Decision-making in Psychology." The chapter "Perception of luck."