Luck, in the context of games, refers to an event whose outcome is not known ahead of time to any of the game’s participants, or an event which arbitrarily selects an outcome.
Note that these clauses do not fully overlap:
While luck is typically seen to run in opposition to the concept of skill, the two concepts are seemingly orthogonal to eachother1 as seen by the existence of games with high levels of both luck and skill, such as Poker, and games with low luck and low skill, such as Tic-Tac-Toe. It can be argued that while luck can coexist with skill in a general sense, it runs in opposition to skill when placed within the same mechanic, as seen with the fact that Poker’s luck (drawing from a deck of cards) is a separate, if related, mechanic to the one that involves skill (bluffing).
Elias, G. S., Garfield, R., & Gutschera, K. R. (n.d.). Characteristics of Games. MIT Press. ↩