Luck is a concept that is used to describe the uncertain outcome of events. It's a subjective judgment based on the perception of the speaker, who assesses the potential outcomes of an event.
The two main factors that influence the perception of luck is the amount of benefit and the perceived likelihood.
For example, calling heads on a coin flip for a bet of 10 cents, and having it turn up heads, would not likely be considered lucky. That is because the outcome is relatively predictable, and the benefit is small. However, if the outcome did have a significant benefit, such as saving a person's life, it would be considered lucky even with the 50% chance. On the other hand, if a number is called on a fifty-sided die and it comes up, even with only a small benefit, it is more likely to be considered lucky because the outcome is quite unlikely.